Fix JValue::operator==(const std::string& other)

It was error in the operator. Return code from std::string::compare was
interpreted in a wrong way

Open-webOS-DCO-1.0-Signed-off-by: Alekseyev Oleksandr <oalekseyev@luxoft.com>

Change-Id: I95bceb8a29233d4f9d4e036d2de07ce5f6c8dfae