Fix compiler warnings for AsyncJson’s _contentLength/_maxContentLength vars (me-no-dev#439)

The type of _contentLength and _maxContentLength should both be unsigned
integer type size_t.

Author: buddydvd

src/AsyncJson.h

@@ -148,11 +148,11 @@ class AsyncCallbackJsonWebHandler: public AsyncWebHandler {
   const String _uri;
   WebRequestMethodComposite _method;
   ArJsonRequestHandlerFunction _onRequest;
-  int _contentLength;
+  size_t _contentLength;
 #ifndef ARDUINOJSON_5_COMPATIBILITY   
   const size_t maxJsonBufferSize;
 #endif
-  int _maxContentLength;
+  size_t _maxContentLength;
 public:
 #ifdef ARDUINOJSON_5_COMPATIBILITY      
   AsyncCallbackJsonWebHandler(const String& uri, ArJsonRequestHandlerFunction onRequest)