Corrected datatype für JSON_MIMETYPE (me-no-dev#396)

* Change JSON MIME type to application/json

* Corrected datatype für JSON_MIMETYPE (Fixes me-no-dev#390)

Author: merlinschumacher

src/AsyncJson.h

@@ -36,7 +36,7 @@
 #define ASYNC_JSON_H_
 #include <ArduinoJson.h>
 
-const char* JSON_MIMETYPE = "application/json";
+constexpr char* JSON_MIMETYPE = "application/json";
 
 /*
  * Json Response