Throws a TypeError when header does not contain exactly two items.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26217
https://github.com/whatwg/fetch/commit/fb8df66cdd69e5a5afcdd7e213875ec8cd299b10

BUG=373120

Review URL: https://codereview.chromium.org/359283002

git-svn-id: svn://svn.chromium.org/blink/trunk@177201 bbb929c8-8fbe-4397-9dbb-9b2b20218538