Set filename to default value if none is provided during file uploads

This prevents 405 errors from occuring during uploads if the file request builder's name
property is somehow nil.
Set application/json Content-Type header for POST and PUT JSON API calls
Add date to default upload name
Simplify non-empty filename method signature
Remove unnecessary temp variable in nonEmptyFilename
Merge pull request #50 from box/stack-bug-fixes

Several fixes for bugs reported on stack overflow