Test XMLHttpRequest overrideMimeType for exceptions.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

Testing overrideMimeType when readyState = 0
PASS req.overrideMimeType("text/plain") did not throw exception.
Testing overrideMimeType when readyState = 1
PASS req.overrideMimeType("text/plain") did not throw exception.
Testing overrideMimeType when readyState = 1
PASS req.overrideMimeType("text/plain") did not throw exception.
Testing overrideMimeType when readyState = 2
PASS req.overrideMimeType("text/plain") did not throw exception.
Testing overrideMimeType when readyState = 3
PASS req.overrideMimeType("text/plain") threw exception InvalidStateError: The object is in an invalid state..
Testing overrideMimeType when readyState = 4
PASS req.overrideMimeType("text/plain") threw exception InvalidStateError: The object is in an invalid state..
PASS successfullyParsed is true

TEST COMPLETE