Check the Content-Type header that is returned from the RPC server. Only
if it is `application/json` the data is supposed to be parsed as JSON.
This gives better reporting if the HTTP server happens to return an error that is
not JSON-formatted, which is the case if it happens at a lower level
before JSON-RPC kicks in.
Before: `Unexpected exception caught during testing: No JSON object could be decoded`
After: `JSONRPC error: non-JSON HTTP response with '403 Forbidden' from server`
Avoid an infinite loop in encoding, by ensuring EncodeDecimal
returns a string. round(Decimal) used to convert it to
float, but it no longer does in python 3.x. Strings are
supported since #6380, so just use that.
Python's httplib does not graciously handle disconnections from the http server, resulting in BadStatusLine errors.
See https://bugs.python.org/issue3566 "httplib persistent connections violate MUST in RFC2616 sec 8.1.4."
This was fixed in Python 3.5.
Work around it for now.
