CP-2244 Add ability to override the response encoding

[evanweible-wf]

Currently, the response encoding is determined from the charset attribute of the content-type response header. If it's missing or isn't recognized, it falls back to LATIN-1 according to RFC 2616. Although the server should always provide an accurate charset attribute, it may be useful to allow the encoding to be explicitly overridden in cases where the requester does not have control of the server.