[feature] Add support for PAWS (RFC 7545) 'spectrum.paws.methodName'

[eshikafe]

Is your feature request related to a problem? Please describe.
Support for PAWS protocol method format spectrum.paws.methodName
should be added to gorilla.

I currently get the following errors when I try to use the PAWS JSON-RPC 2.0 format for the method name.

"jsonrpc": "2.0",
"error": {
        "code": -32000,
        "message": "rpc: service/method request ill-formed: \"spectrum.paws.init\"",
        "data": null
    },

Describe the solution you'd like
spectrum.paws should be allowed as a service name as shown below:

s := rpc.NewServer()
s.RegisterCodec(json2.NewCodec(), "application/json")
s.RegisterService(new(SpectrumPaws), "spectrum.paws")

Describe alternatives you've considered
Create a fork of this repo and update the map.go file to support spectrum.paws.Method