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")`.

jsonrpc2.go

@@ -32,7 +32,7 @@ type Error struct {
 	Data    *json.RawMessage `json:"data,omitempty"`
 }
 
-// SetError sets e.Data to the JSON representation of v. If JSON
+// SetError sets e.Data to the JSON encoding of v. If JSON
 // marshaling fails, it panics.
 func (e *Error) SetError(v interface{}) {
 	b, err := json.Marshal(v)