Allow overriding of all headers from passed environment hash

Allow REMOTE_ADDR, HTTP_HOST and HTTP_USER_AGENT to be overridden from
the environment passed into `ActionDispatch::TestRequest.new`.

Fixes #11590

(cherry picked from commit 4db0637d556ab11c55de6afcaa267147a032a74d)

Conflicts:
	actionpack/CHANGELOG.md