fix: cast to proper interface (#124)

Fixes #123

Author: codyoss

google-cloud-core/src/main/java/com/google/cloud/ServiceOptions.java

@@ -218,7 +218,7 @@ public B setCredentials(Credentials credentials) {
       }
 
       if (this.quotaProjectId == null && credentials instanceof QuotaProjectIdProvider) {
-        this.quotaProjectId = ((ServiceAccountCredentials) credentials).getQuotaProjectId();
+        this.quotaProjectId = ((QuotaProjectIdProvider) credentials).getQuotaProjectId();
       }
       return self();
     }