Fix for XBlock/Studio/time bug

@dmitchell @cpennington I was using XBlocks in Studio, and getting funny import/export errors with XML content. This fixes it. I do not understand the code path around this basically at all. The fix was purely mechanical -- I saw strings going in and matching exceptions, so I made it handle strings. I'd appreciate a more detailed look-over to see if this is sane. 
probably should be `basestring` so unicode doesn't trip it.