Fix nextSuccessfulRegistrationResponse

* Fix issue where nextSuccessfulRegistrationResponse could be used for
other types of POST requests which should only be limited to players.
* This fixes some flacky tests with InAppMessageIntegrationTests as
the response was used for impressions and was discarded.

Author: jkasten2

OneSignalSDK/unittest/src/test/java/com/onesignal/ShadowOneSignalRestClient.java

@@ -311,7 +311,7 @@ private static void mockPost(String url, JSONObject jsonBody, OneSignalRestClien
 
       String retJson;
 
-      if (nextSuccessfulRegistrationResponse != null) {
+      if (nextSuccessfulRegistrationResponse != null && url.contains("players")) {
          retJson = nextSuccessfulRegistrationResponse;
          nextSuccessfulRegistrationResponse = null;
       }