Gracefully handle request for binary data as plain

This will return a 406 instead of a 500 in the event that a user
requests binary data but provides text/plain as the content type
that they accept.

Change-Id: I003c8306138db7d8d53d30c2e83f29a51aa3759d
Closes-Bug: #1220957
Merge "Gracefully handle request for binary data as plain"