Fixed broken BlobField/BinaryField in MySQL
This will need sign off and review from @coleifer before merge. I'm about 90% happy with the code, but there could be some edge cases I haven't considered.
 I'm leaning towards killing off the binary field altogether.
 @coleifer Yeah I just updated the original issue with my conclusion, lemme know your thoughts
 Closing as per #818 

Fixes #818
