Replace HttpResponse mimetype with content_type as deprecated
replacing missed mimetype with content_type
adding space after comma
replacing content_type 'json' by 'application/json'
HttpRequest cleanup

making sure Content-Type is set to correct value
while passing JSON data, see RFC 4627 http://www.ietf.org/rfc/rfc4627.txt
moving custom HttpResponses to the omeroweb.http