Add POST support to XHR as well as .status and statusText support

Also fixed all the XHR tests to actually run and work
I learned from elliot that the it function has to
take a "done" argument to be treated asynchronously.

My utf8 conversion is a hack, but we deleted
window.TextEncoder (it was a module) when we forked.
We could resurrect TextEncoder (and probably should)
but I've left that for a separate change.

I also augmented sky_server to have a special /echo_post
handler so we can test POST requests.

R=esprehn@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/5ef81d249b841f44c9593ffb0158d64c7e0febfc

Review URL: https://codereview.chromium.org/810523002