Making JSON-RPC accept more content types

According to the RFC for JSON-RPC over HTTP at
http://www.jsonrpc.org/historical/json-rpc-over-http.html
"Content-Type SHOULD be 'application/json-rpc' but MAY be
'application/json' or 'application/jsonrequest'"

This patch fixes HTTPJsonRpcHandler to allow for any of those three
content types when accepting JSON-RPC HTTP requests.

This also fixes compatibility with the popular Python library jsonrpclib
which uses the application/json-rpc content type.
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