Check for JSONObject.NULL before casting to String [Fixes #209]

Author: jhansche

Parse/src/main/java/com/parse/ParseUser.java

@@ -141,7 +141,11 @@ public Builder newBuilder() {
     }
 
     public String sessionToken() {
-      return (String) get(KEY_SESSION_TOKEN);
+      Object value = get(KEY_SESSION_TOKEN);
+      if (!(value instanceof String)) {
+        return null;
+      }
+      return (String) value;
     }
 
     @SuppressWarnings("unchecked")

Parse/src/test/java/com/parse/ParseUserTest.java

@@ -1091,6 +1091,15 @@ public void testDontOverwriteSessionTokenForCurrentUser() throws Exception {
     assertNull(user.getString("key2"));
   }
 
+  @Test
+  public void testSessionTokenFromNull() throws Exception {
+    ParseUser.State userState = new ParseUser.State.Builder()
+            .put("sessionToken", JSONObject.NULL)
+            .build();
+    ParseUser user = ParseObject.from(userState);
+    assertNull(user.getSessionToken());
+  }
+
   //endregion
 
   //region testUnlinkFromAsync