Don't clobber the JSON MIME type.

simple_datatables was preventing Rails from parsing POST messages with a JSON
body.  The reason was that Mime::Type.register will associate a symbol with a
mime type for both content generation (outbound HTTP responses, for instance)
and content parsing (inbound HTTP requests), so when the middleware retrieved
the identifying symbol associated with the application/json datatype, it got
back :datatables, which it ignored. Mime::Type.register_alias will associate
the symbol with the mime type for content generation only, so parsing the JSON
body into a parameters array still works.