Fix OAuth2 issue importing a new project from GitHub and GitLab

It appears that the GitLab OAuth2 client options were converted to strings instead of symbols when merged with the default options (i.e. `{}.merge(github_options)`). As a result, the OAuth2 defaults were being used. For example, the OAuth2 client options would have a key with `authorize_url` and `:authorize_url`, but the former was never used. As a result, the OAuth2 client would always use the wrong URL to talk to GitHub.

Note that this bug should also have affected GitLab, but not Bitbucket: The OAuth client is careful to convert all keys to symbols.

Closes #1268

See merge request !425