Split jsonrpc2.go file into multiple files (#65)

This merge request moves some of the contents from the jsonrpc2.go file
into their own designated file. The new files being introduced
(excluding test files) are as follows:

* conn.go
* request.go
* response.go

The motive of this change is to make it easier to navigate the code.
Without this change, the jsonrpc2.go file is 813 lines of code.

request_test.go

@@ -0,0 +1,65 @@
+package jsonrpc2_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"reflect"
+	"testing"
+
+	"github.com/sourcegraph/jsonrpc2"
+)
+
+func TestRequest_MarshalJSON_jsonrpc(t *testing.T) {
+	b, err := json.Marshal(&jsonrpc2.Request{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := `{"id":0,"jsonrpc":"2.0","method":""}`; string(b) != want {
+		t.Errorf("got %q, want %q", b, want)
+	}
+}
+
+func TestRequest_MarshalUnmarshalJSON(t *testing.T) {
+	null := json.RawMessage("null")
+	obj := json.RawMessage(`{"foo":"bar"}`)
+	tests := []struct {
+		data []byte
+		want jsonrpc2.Request
+	}{
+		{
+			data: []byte(`{"id":123,"jsonrpc":"2.0","method":"m","params":{"foo":"bar"}}`),
+			want: jsonrpc2.Request{ID: jsonrpc2.ID{Num: 123}, Method: "m", Params: &obj},
+		},
+		{
+			data: []byte(`{"id":123,"jsonrpc":"2.0","method":"m","params":null}`),
+			want: jsonrpc2.Request{ID: jsonrpc2.ID{Num: 123}, Method: "m", Params: &null},
+		},
+		{
+			data: []byte(`{"id":123,"jsonrpc":"2.0","method":"m"}`),
+			want: jsonrpc2.Request{ID: jsonrpc2.ID{Num: 123}, Method: "m", Params: nil},
+		},
+		{
+			data: []byte(`{"id":123,"jsonrpc":"2.0","method":"m","sessionId":"session"}`),
+			want: jsonrpc2.Request{ID: jsonrpc2.ID{Num: 123}, Method: "m", Params: nil, ExtraFields: []jsonrpc2.RequestField{{Name: "sessionId", Value: "session"}}},
+		},
+	}
+	for _, test := range tests {
+		var got jsonrpc2.Request
+		if err := json.Unmarshal(test.data, &got); err != nil {
+			t.Error(err)
+			continue
+		}
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("%q: got %+v, want %+v", test.data, got, test.want)
+			continue
+		}
+		data, err := json.Marshal(got)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if !bytes.Equal(data, test.data) {
+			t.Errorf("got JSON %q, want %q", data, test.data)
+		}
+	}
+}