Fix the mistaken deletion in cache (#241)

Merging the change.

Author: hrishileanplum

Leanplum-SDK/Classes/Features/Variables/LPVarCache.m

@@ -488,6 +488,7 @@ - (void)applyVariableDiffs:(NSDictionary *)diffs_
         if (messages_) {
             // Store messages.
             self.messageDiffs = messages_;
+            self.messages = [NSMutableDictionary dictionary];
             for (NSString *name in messages_) {
                 NSDictionary *messageConfig = messages_[name];
                 NSMutableDictionary *newConfig = [messageConfig mutableCopy];