metadata: type conversion happens at parsing, not at writing

These test cases were writing assuming they had to transform the data
format.  That is no longer the case.  Going forward, the parsing process
converts everything to a standardized format.  That will hopefully be
enforceable by the JSON Schema in the future.

tests/metadata.TestCase

@@ -720,7 +720,7 @@ class MetadataTest(unittest.TestCase):
         build.versionCode = '0'  # taken from fdroidserver/import.py
         build.disable = 'Generated by import.py ...'
         build.commit = 'Unknown'
-        build.gradle = [True]
+        build.gradle = ['yes']
         app['Builds'] = [build]
 
         fdroidserver.metadata.write_yaml(mf, app)
@@ -745,7 +745,7 @@ class MetadataTest(unittest.TestCase):
                     disable: Generated by import.py ...
                     commit: Unknown
                     gradle:
-                      - true
+                      - yes
 
                 AutoUpdateMode: None
                 UpdateCheckMode: Tags
@@ -1886,10 +1886,6 @@ class MetadataTest(unittest.TestCase):
         app = metadata.App({'Builds': [metadata.Build({'rm': []})]})
         self.assertEqual(dict(), metadata._app_to_yaml(app)['Builds'][0])
 
-    def test_app_to_yaml_build_list_string(self):
-        app = metadata.App({'Builds': [metadata.Build({'rm': 'one'})]})
-        self.assertEqual({'rm': 'one'}, metadata._app_to_yaml(app)['Builds'][0])
-
     def test_app_to_yaml_build_list_one(self):
         app = metadata.App({'Builds': [metadata.Build({'rm': ['one']})]})
         self.assertEqual({'rm': ['one']}, metadata._app_to_yaml(app)['Builds'][0])