Fix unmarshaling of HTTP POST responses.

If connecting to a bitcoin RPC server as an HTTP POST client, full
response objects rather than just the raw result bytes were being
passed to the specific result unmarshalers, causing unmarshal errors
for the incorrect JSON types.  To fix this, first unmarshal the
response body into a rawResponse, and pass only the raw result bytes
(or an error) to the specific handlers.

This was reported by nskelsey on IRC.

ok @davecgh