Fix sendrawtransaction for websockets.

This fixes two issues: first, the sendrawtransaction handler had an
extra character in the key in the websocket handler map, preventing
the handler from never running.  Second, a nil pointer dereference was
removed from the handler.

This change fixes the minedtx notifications for btcwallet, since the
websocket-handler now runs instead of falling back to the legacy RPC
handler.