[Fetch API] Don't append empty Content-Type header to the response.

The fetch spec has changed not to append empty Content-Type header.
https://github.com/slightlyoff/ServiceWorker/issues/363#event-149733415

TEST=run_webkit_tests.py http/tests/serviceworker/response.html
BUG=408929

Review URL: https://codereview.chromium.org/526963002

git-svn-id: svn://svn.chromium.org/blink/trunk@181169 bbb929c8-8fbe-4397-9dbb-9b2b20218538