Make sure missing credentials returns the right error.

Signed-off-by: David Calavera <david.calavera@gmail.com>

osxkeychain/osxkeychain_darwin_test.go

@@ -35,3 +35,11 @@ func TestOSXKeychainHelper(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestMissingCredentials(t *testing.T) {
+	helper := New()
+	_, _, err := helper.Get("https://adsfasdf.wrewerwer.com/asdfsdddd")
+	if err != credentials.ErrCredentialsNotFound {
+		t.Fatal("exptected ErrCredentialsNotFound, got %v", err)
+	}
+}