[whatwg/xhr] Fix overrideMimeType() again (#174)

Reset override MIME type when open() is invoked.

Actually make overriding the charset parameter work (setting override MIME type to the "MIME type portion" (now essence) as done previously would always erase it.

Tests: https://github.com/w3c/web-platform-tests/pull/8449.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/xhr/pull/174

-- Commit Summary --

  * Fix overrideMimeType() again

-- File Changes --

    M xhr.bs (59)

-- Patch Links --

https://github.com/whatwg/xhr/pull/174.patch
https://github.com/whatwg/xhr/pull/174.diff

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https://github.com/whatwg/xhr/pull/174