OS: Fix revert_resize_server method name.

Fixes an issue where calling revert_resize on the OpenStack Server
object would fail with a method name error.

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Updates the revert_resize_server request methods to match the filename.
This also makes it match the method name we call in the
compute/server.rb module (revert_resize_server instead of
revert_resized_server).

Adds a test case which should guard against the above issue.