Enable and fix sign-conversion warnings

Created by: armandas

We're using -Wsign-conversion GCC option and we're getting errors from ArduinoJson. The errors are caused by the following template code in RamString.hpp:

template <int N>
struct IsString<char[N]> : true_type {};

template <int N>
struct IsString<const char[N]> : true_type {};

template <int N>
inline SizedRamString adaptString(char s[N]) {
  return SizedRamString(s, strlen(s));
}

All other places use template <size_t N>.

Would it be possible to add -Wsign-conversion to the compiler options and fix the reported issues? I could create a PR.