Correct context uses

`doAPI`'s `context` does not work due to `WithContext` immutability.
`Authenticate` and `RegisterApp` does not use `context` although they
needs `context`, so their `context` does not work also. This commit fixes
them and add test cases.

apps_test.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 )
 
 func TestRegisterApp(t *testing.T) {
@@ -41,3 +42,25 @@ func TestRegisterApp(t *testing.T) {
 		t.Fatalf("want %q but %q", "bar", app.ClientSecret)
 	}
 }
+
+func TestRegisterAppWithCancel(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(3 * time.Second)
+		fmt.Fprintln(w, `{"client_id": "foo", "client_secret": "bar"}`)
+		return
+	}))
+	defer ts.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	go cancel()
+	_, err := RegisterApp(ctx, &AppConfig{
+		Server: ts.URL,
+		Scopes: "read write follow",
+	})
+	if err == nil {
+		t.Fatalf("should be fail: %v", err)
+	}
+	if want := "Post " + ts.URL + "/api/v1/apps: context canceled"; want != err.Error() {
+		t.Fatalf("want %q but %q", want, err.Error())
+	}
+}