Adds content to the success message 'mock' (thephpleague#225)

Author: frankverhoeven

tests/Mock/RestGenericSubscriptionSuccess.txt

@@ -5,3 +5,5 @@ Paypal-Debug-Id: 217a9ddefd384
 SERVER_INFO: identitysecuretokenserv:v1.oauth2.token&CalThreadId=91&TopLevelTxnStartTime=146fbfe679a&Host=slcsbidensectoken502.slc.paypal.com&pid=29059
 CORRELATION-ID: 217a9ddefd384
 Date: Thu, 03 Jul 2014 11:31:32 GMT
+
+{}