oauth2: long if condition in providerAuthHeaderWorks replaced with loop

Long if condition replaced with loop.

Related to issue #41.

Change-Id: Ib5b88ce2ee2841e9b6c24d78f93bb027141bf678
Reviewed-on: https://go-review.googlesource.com/7290
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Burcu Dogan <jbd@google.com>

oauth2_test.go

@@ -407,3 +407,20 @@ func TestConfigClientWithToken(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func Test_providerAuthHeaderWorks(t *testing.T) {
+	for _, p := range brokenAuthHeaderProviders {
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+		p := fmt.Sprintf("%ssomesuffix", p)
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+	}
+	p := "https://api.not-in-the-list-example.com/"
+	if !providerAuthHeaderWorks(p) {
+		t.Errorf("URL: %s found in list", p)
+	}
+
+}