Fix conversion to WebServiceWorkerResponse headers.

When converting from Response headers to WebServiceWorkerResponse headers the fields aren't coalesced.  This CL adds coalescing by using appendHeader instead of setHeader.

A similar CL for WebServiceWorkerRequest headers is here: https://codereview.chromium.org/578763003

BUG=415109

Review URL: https://codereview.chromium.org/564773004

git-svn-id: svn://svn.chromium.org/blink/trunk@182364 bbb929c8-8fbe-4397-9dbb-9b2b20218538