Fix Client URL Prepending For GraphQL Endpoint on Enterprise

[asher-goldberg]

We noticed a potential bug when trying to use the library with the V4/GraphQL API and our Enterprise server.

The URL prepending that occurs in the Client class for GHE always sets the prefix to /api/{version} but for the GraphQL/v4 endpoint the prepend needs to be /api/graphql. This leads to a Not Found Exception on all queries. The class defaults to v3 if no version is provided so we can't just send a blank to get /api

$builder->addPlugin(new PathPrepend(sprintf('/api/%s', $this->getApiVersion())));

results in a call to https://{enterprise_url}/api/v3/graphql which is not the real endpoint so you end up with a not found exception.

  Uncaught Github\Exception\RuntimeException: Not Found