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.

response_test.go

@@ -0,0 +1,110 @@
+package jsonrpc2_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"reflect"
+	"testing"
+
+	"github.com/sourcegraph/jsonrpc2"
+)
+
+func TestResponse_MarshalJSON_jsonrpc(t *testing.T) {
+	null := json.RawMessage("null")
+	b, err := json.Marshal(&jsonrpc2.Response{Result: &null})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := `{"id":0,"result":null,"jsonrpc":"2.0"}`; string(b) != want {
+		t.Errorf("got %q, want %q", b, want)
+	}
+}
+
+func TestResponseMarshalJSON_Notif(t *testing.T) {
+	tests := map[*jsonrpc2.Request]bool{
+		{ID: jsonrpc2.ID{Num: 0}}:                   true,
+		{ID: jsonrpc2.ID{Num: 1}}:                   true,
+		{ID: jsonrpc2.ID{Str: "", IsString: true}}:  true,
+		{ID: jsonrpc2.ID{Str: "a", IsString: true}}: true,
+		{Notif: true}: false,
+	}
+	for r, wantIDKey := range tests {
+		b, err := json.Marshal(r)
+		if err != nil {
+			t.Fatal(err)
+		}
+		hasIDKey := bytes.Contains(b, []byte(`"id"`))
+		if hasIDKey != wantIDKey {
+			t.Errorf("got %s, want contain id key: %v", b, wantIDKey)
+		}
+	}
+}
+
+func TestResponseUnmarshalJSON_Notif(t *testing.T) {
+	tests := map[string]bool{
+		`{"method":"f","id":0}`:   false,
+		`{"method":"f","id":1}`:   false,
+		`{"method":"f","id":"a"}`: false,
+		`{"method":"f","id":""}`:  false,
+		`{"method":"f"}`:          true,
+	}
+	for s, want := range tests {
+		var r jsonrpc2.Request
+		if err := json.Unmarshal([]byte(s), &r); err != nil {
+			t.Fatal(err)
+		}
+		if r.Notif != want {
+			t.Errorf("%s: got %v, want %v", s, r.Notif, want)
+		}
+	}
+}
+
+func TestResponse_MarshalUnmarshalJSON(t *testing.T) {
+	null := json.RawMessage("null")
+	obj := json.RawMessage(`{"foo":"bar"}`)
+	tests := []struct {
+		data  []byte
+		want  jsonrpc2.Response
+		error bool
+	}{
+		{
+			data: []byte(`{"id":123,"result":{"foo":"bar"},"jsonrpc":"2.0"}`),
+			want: jsonrpc2.Response{ID: jsonrpc2.ID{Num: 123}, Result: &obj},
+		},
+		{
+			data: []byte(`{"id":123,"result":null,"jsonrpc":"2.0"}`),
+			want: jsonrpc2.Response{ID: jsonrpc2.ID{Num: 123}, Result: &null},
+		},
+		{
+			data:  []byte(`{"id":123,"jsonrpc":"2.0"}`),
+			want:  jsonrpc2.Response{ID: jsonrpc2.ID{Num: 123}, Result: nil},
+			error: true, // either result or error field must be set
+		},
+	}
+	for _, test := range tests {
+		var got jsonrpc2.Response
+		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 {
+			if test.error {
+				continue
+			}
+			t.Error(err)
+			continue
+		}
+		if test.error {
+			t.Errorf("%q: expected error", test.data)
+			continue
+		}
+		if !bytes.Equal(data, test.data) {
+			t.Errorf("got JSON %q, want %q", data, test.data)
+		}
+	}
+}