UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.29 Safari/537.36

Steps to reproduce the problem:
1. Created a simple test web server using flask and python3.6:
```python
from flask import Flask, make_response

app = Flask(__name__)

@app.route('/header', methods=['GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS'])
def header():
    resp = make_response('', 200)
    resp.headers['Access-Control-Allow-Headers'] = 'content-type, token'
    resp.headers['Access-Control-Allow-Methods'] = 'POST, GET, OPTIONS, PUT, PATCH, DELETE'
    resp.headers['Access-Control-Allow-Origin'] = '*'
    resp.headers['Content-Type'] = 'text/plain; charset=utf-8'
    resp.headers['Token'] = 'yes'
    return resp

@app.route('/noheader', methods=['GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS'])
def noheaders():
    resp = make_response('', 200)
    resp.headers['Access-Control-Allow-Headers'] = 'content-type'
    resp.headers['Access-Control-Allow-Methods'] = 'POST, GET, OPTIONS, PUT, PATCH, DELETE'
    resp.headers['Access-Control-Allow-Origin'] = '*'
    resp.headers['Content-Type'] = 'text/plain; charset=utf-8'
    resp.headers['Token'] = 'yes'
    return resp

app.run('127.0.0.1', port=3333)
```
2. Open chrome and access a web page that is NOT the 127.0.0.1:3333 address.
3. Open Developer tools
4. Switch to console
5. Enter the following in console
```js
let headers = {
  Accept: 'text/plain; charset=utf-8',
  'Content-Type': 'text/plain; charset=utf-8',
};
let endpoints = ['header','noheader'];
['get','post'].map(function(method) {
	let rtn = [];
	let settings = { method, headers };
	for (let i = 0; i < endpoints.length; i += 1) {
		let settings = { method, headers };
                rtn.push(fetch('http://127.0.0.1:3333/' + endpoints[i], settings ).then(function(response) {
		    return {method:method, endpoint:endpoints[i], resp_ok: response.ok, resp_content_type: response.headers.get('content-type'), resp_token_lower: response.headers.get('token'), resp_token_cap: response.headers.get('token')};
    	        }));
        };
	return rtn;
});
```
6. Inspecting the output of the nested array.  You'll find all resp_token_cap & resp_token_lower show null.  This only happens when cors is in effect.
[
  [
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "header",
        method: "get",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: null,
        resp_token_lower: null
      }
    },
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "noheader",
        method: "get",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: null,
        resp_token_lower: null
      }
    }
  ],
  [
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "header",
        method: "post",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: null,
        resp_token_lower: null
      }
    },
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "noheader",
        method: "post",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: null,
        resp_token_lower: null
      }
    }
  ]
]

What is the expected behavior?
Pass all headers to fetch response object,  Like Firefox 45.7.0 does.

[
  [
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "header",
        method: "get",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: 'yes',
        resp_token_lower: 'yes'
      }
    },
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "noheader",
        method: "get",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: 'yes',
        resp_token_lower: 'yes'
      }
    }
  ],
  [
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "header",
        method: "post",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: 'yes',
        resp_token_lower: 'yes'
      }
    },
    Promise: {
      [[PromiseStatus]]: "resolved",
      [[PromiseValue]]: {
        endpoint: "noheader",
        method: "post",
        resp_content_type: "text/plain; charset=utf-8",
        resp_ok: true,
        resp_token_cap: 'yes',
        resp_token_lower: 'yes'
      }
    }
  ]
]

What went wrong?
For some reason when CORS is involved in a FETCH request it strips headers.

Did this work before? N/A 

Chrome version: 59.0.3071.29  Channel: beta
OS Version: 10.0
Flash Version: