Make GenericUrl's #equals pass static analysis (googleapis#440)

We'll start matching this with a new error-prone (https://github.com/google/error-prone) check soon otherwise: it looks like this is comparing two different getters.

google-http-client/src/main/java/com/google/api/client/http/GenericUrl.java

@@ -183,7 +183,7 @@ public boolean equals(Object obj) {
     }
     GenericUrl other = (GenericUrl) obj;
     // TODO(yanivi): optimize?
-    return build().equals(other.toString());
+    return build().equals(other.build());
   }
 
   @Override