add Content-Type and Accept headers

According to the jsonrpc 1.2 spec, as well as the HTTP transport
proposal, good citizen implementations of jsonrpc are required to
include these headers. Servers "should" refuse clients not specifying
these headers as well.
Add Matt Broadstone to the credits section
Make sure we don't overwrite user-defined headers when configuring RFC-defined ones
Merge http-headers
Test private methods protection mechanism