Allow PrettyPrintResponses to be chosen per Response

[bgrant0607]

We want to pretty-print responses by default, but not to pretty-print responses to requests sent by our client library, since they won't be directly looked at by humans. Right now, PrettyPrintResponses is a global, which isn't thread-friendly to change on a per-request/response basis. What I'd prefer is a field in Response that is initialized from the global but which could be changed just for that response (SetPrettyPrint(bool)).

[bgrant0607]

FYI, this is low priority. We're not yet using Response.WriteEntity or WriteAsJson.