Always omit params member from request when empty

With this commit, the JSON encoding of `Request` always omits the params
member when calling `Conn.Call`, `Conn.DispatchCall`, or `Conn.Notify`
with the `params` argument set to `nil`. This change also removes the
`OmitNilParams` call option that was added in commit 8012d496 (#62).

As of this commit, if users desire to send a JSON-RPC request with a
`params` value of `null`, then they may do so by explicitly setting the
`params` argument of `Conn.Call`/`Conn.DispatchCall`/`Conn.Notify` to
`json.RawMessage("null")`.

call_opt.go

@@ -46,12 +46,3 @@ func StringID() CallOption {
 		return nil
 	})
 }
-
-// OmitNilParams returns a call option that instructs requests to omit params
-// values of nil instead of JSON encoding them to null.
-func OmitNilParams() CallOption {
-	return callOptionFunc(func(r *Request) error {
-		r.OmitNilParams = true
-		return nil
-	})
-}