Fix custom github repositories failing to parse

This commit resolves a type conflicts, which caused exception when parsing
release information of packages, hosted on GitHub.

As a result packages from Github repositories, using tag based releases were
not parsed and thus

1. not published to "Package Control: Install Packages" quick panel.
2. not added to cache, causing them to be re-queried each time the quick panel
   was triggered (see #1645).

Note:

Only Package Control client is effected, which tries to reduce amount of API
calls, by not downloading commit info of each release tag.

package_control/clients/github_client.py

@@ -187,7 +187,7 @@ def _get_releases(user_repo, tag_prefix=None, page_size=1000):
             version, tag, tag_url = release
 
             if is_client:
-                timestamp = 0
+                timestamp = '1970-01-01 00:00:00'
             else:
                 tag_info = self.fetch_json(tag_url)
                 timestamp = tag_info['commit']['committer']['date'][0:19].replace('T', ' ')

package_control/providers/repository_provider.py

@@ -787,8 +787,9 @@ def has_broken_release():
                 # Extract a date from the newest release
                 date = '1970-01-01 00:00:00'
                 for release in info['releases']:
-                    if 'date' in release and release['date'] > date:
-                        date = release['date']
+                    release_date = release.get('date')
+                    if release_date and isinstance(release_date, str) and release_date > date:
+                        date = release_date
                 info['last_modified'] = date
 
             output[info['name']] = info