Trailing whitespace in XHR response headers is exposed

[pshaughn]

WPT test xhr/headers-normalize-response asks the server to send back a custom header ending in a space or tab, and unexpectedly can see that trailing space or tab in the returned header. I don't know which code is responsible: this may need a call on whether to strip immediately when storing the headers from the network HTTP response, or to keep the spaces/tabs at that time and strip them when returning them to Javascript getResponseHeader.

Regarding WPT results, Firefox and Safari pass these subtests and fail different subtests which Servo does pass (Servo's doing the right thing on NUL bytes).