Propagate graphql url to installation scoped client (#184)

Author: dennisgranath

src/main/java/com/spotify/github/v3/clients/GitHubClient.java

@@ -360,7 +360,7 @@ public GitHubClient withScopeForInstallationId(final int installationId) {
     return new GitHubClient(
         client,
         baseUrl,
-        null,
+        graphqlUrl.orElse(null),
         null,
         privateKey,
         appId,