add test for allowing to write placeholder values to yaml metadata files

tests/metadata.TestCase

@@ -165,6 +165,51 @@ class MetadataTest(unittest.TestCase):
             with self.assertRaises(MetaDataException):
                 fdroidserver.metadata.parse_yaml_metadata(mf, {})
 
+    def test_write_yaml_with_allow_placeholder_values(self):
+        mf = io.StringIO()
+
+        app = fdroidserver.metadata.App()
+        app.Categories = ['None']
+        app.SourceCode = "https://gitlab.com/fdroid/fdroidclient.git"
+        app.IssueTracker = "https://gitlab.com/fdroid/fdroidclient/issues"
+        app.RepoType = 'git'
+        app.Repo = 'https://gitlab.com/fdroid/fdroidclient.git'
+        app.AutoUpdateMode = 'None'
+        app.UpdateCheckMode = 'Tags'
+        build = fdroidserver.metadata.Build()
+        build.versionCode = 'Unknown'
+        build.versionName = 'Unknown'
+        build.disable = 'Generated by import.py ...'
+        build.commit = 'Unknown'
+        build.gradle = [True]
+        app.builds = [build]
+
+        fdroidserver.metadata.write_yaml(mf, app,
+                                         allow_placeholder_values=True)
+
+        mf.seek(0)
+        self.assertEqual(mf.read(), textwrap.dedent("""\
+            Categories:
+              - None
+            License: Unknown
+            SourceCode: https://gitlab.com/fdroid/fdroidclient.git
+            IssueTracker: https://gitlab.com/fdroid/fdroidclient/issues
+
+            RepoType: git
+            Repo: https://gitlab.com/fdroid/fdroidclient.git
+
+            Builds:
+              - versionName: Unknown
+                versionCode: '?'
+                disable: Generated by import.py ...
+                commit: Unknown
+                gradle:
+                  - true
+
+            AutoUpdateMode: None
+            UpdateCheckMode: Tags
+            """))
+
 
 if __name__ == "__main__":
     os.chdir(os.path.dirname(__file__))