Art/test roundtrip mime

Added a test (currently suppressed, because it fails) to test round-tripping of MIME message. 

Consolidated the sample MIME messages that were defined in two unrelated test cases (written by different developers, I suppose)

I think the sample MIME messages are better stored as bytes rather than strings, but I'm not sure how best to do this - source code only really handles text. Can we check in a binary file and read it into test code in the AndroidTestCase world?