fix incorrect getHeaderField null mocking

The real getHeaderField supports returning null when the key is not set.
Correcting this fixes failing HTTPClientTest

Author: jkasten2

OneSignalSDK/onesignal/core/src/test/java/com/onesignal/core/internal/http/MockHttpConnectionFactory.kt

@@ -42,8 +42,8 @@ internal class MockHttpConnectionFactory(
         override fun connect() {
         }
 
-        override fun getHeaderField(name: String): String {
-            return mockResponse.mockProps[name]!!
+        override fun getHeaderField(name: String): String? {
+            return mockResponse.mockProps[name]
         }
 
         @Throws(IOException::class)