Allow mixed configuration in compilation units (issue #809)

test/StaticMemoryPool/alloc.cpp

@@ -5,7 +5,7 @@
 #include <ArduinoJson/Memory/StaticMemoryPool.hpp>
 #include <catch.hpp>
 
-using namespace ArduinoJson::Internals;
+using namespace ARDUINOJSON_NAMESPACE;
 
 static bool isAligned(void *ptr) {
   const size_t mask = sizeof(void *) - 1;