Do not include query params in callback URLs

In order to be compatible with GitHub Integration's Oauth flow the callback URL
must match the same one provided in the integration's settings page. The current
`callback_url` method includes any query params received previously, which
causes a mismatch, and GitHub returns "406 Not Accepted" with an error message:

```
(github) Callback phase initiated.
(github) Authentication failure! invalid_credentials: OAuth2::Error,
redirect_uri_mismatch: The redirect_uri MUST match the registered callback URL
for this application.
error=redirect_uri_mismatch&error_description=The+redirect_uri+MUST+match+
the+registered+callback+URL+for+this+application.&error_uri=https%3A%2F%2
Fdeveloper.github.com%2Fv3%2Foauth%2F%23redirect-uri-mismatch2
```

For more information:
https://developer.github.com/early-access/integrations/user-identification-authorization

Author: volmer

lib/omniauth/strategies/github.rb

@@ -69,6 +69,10 @@ def email_access_allowed?
         scopes = options['scope'].split(',')
         (scopes & email_scopes).any?
       end
+
+      def callback_url
+        full_host + script_name + callback_path
+      end
     end
   end
 end

spec/omniauth/strategies/github_spec.rb

@@ -149,4 +149,13 @@
       expect(subject.info['urls']['GitHub']).to eq('http://enterprise/me')
     end
   end
+
+  describe '#callback_url' do
+    it 'is a combination of host, script name, and callback path' do
+      allow(subject).to receive(:full_host).and_return('https://example.com')
+      allow(subject).to receive(:script_name).and_return('/sub_uri')
+
+      expect(subject.callback_url).to eq('https://example.com/sub_uri/auth/github/callback')
+    end
+  end
 end