Send a given cookie name only once per fastboot response

[vincent99]

Currently, every time something calls write() a new Set-Cookie header gets added for fastboot. So if you do:

cookies.write('foo', 'value1');
cookies.write('foo', 'value2');

The response headers are sent as:

Set-Cookie: foo=value1
Set-Cookie: foo=value2

And all that is sent in the response back to the client/browser, which then ignores all but the last set for a given key. ember-simple-auth in particular does several sets of the same key and ends up constantly sending multiple values for the same cookie.

This PR changes write() -> _writeFastBootCookie(name, value) to look for an existing set-cookie header for name and replace the existing entry's value with the new one if found, instead of always appending a new entry.