FetchResponse should set its internal response text encoding name

FetchResponse should set its internal response text encoding name
https://bugs.webkit.org/show_bug.cgi?id=181284

Patch by Youenn Fablet <youenn@apple.com> on 2018-01-04
Reviewed by Alex Christensen.

LayoutTests/imported/w3c:

* web-platform-tests/service-workers/service-worker/fetch-event.https-expected.txt:

Source/WebCore:

Covered by rebased test.

* Modules/fetch/FetchResponse.cpp:
(WebCore::FetchResponse::create): Set response text encoding based on content type charset.

Source/WebKit:

* WebProcess/Storage/ServiceWorkerClientFetch.cpp:
(WebKit::ServiceWorkerClientFetch::didReceiveResponse): Set default encoding to UTF-8.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226410 268f45cc-cd09-0410-ab3c-d52691b4dbfc