Add tests for this function.

The actual sending of the request is still done in I_RpcSendReceive though.

Disallow the server from calling I_RpcSend and I_RpcReceive to allow 
simplification of the code. The releasing of the connection is now done 
in I_RpcFreeBuffer.

Implement I_RpcNegotiateTransferSyntax.

Only call IRpcChannelBuffer_FreeBuffer in NdrProxyFreeBuffer if 
fBufferValid is set.