Fix cpp-restbed-server json field serialization

Created by: lwlee2608

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fix from

std::string DemoSchema::toJsonString()
{
	std::stringstream ss;
	ptree pt;
	pt.put("dollarsign", m_dollarsign); // should be "$dollarsign"
	pt.put("paragraph", m_paragraph); // should be "§paragraph"
	pt.put("_operator", m__operator); // should be "operator"
	pt.put("Firstupper", m_Firstupper); // alright
	pt.put("Noupper", m_Noupper); // should be "noupper"
	pt.put("MidUpper", m_MidUpper); // should be "midUpper"
	pt.put("ALLUPPER", m_ALLUPPER); // alright
	write_json(ss, pt, false);
	return ss.str();
}

to the following

std::string DemoSchema::toJsonString()
{
        std::stringstream ss; 
        ptree pt; 
        pt.put("$dollarsign", m_dollarsign);
        pt.put("§paragraph", m_paragraph);
        pt.put("operator", m__operator);
        pt.put("Firstupper", m_Firstupper);
        pt.put("noupper", m_Noupper);
        pt.put("midUpper", m_MidUpper);
        pt.put("ALLUPPER", m_ALLUPPER);
        write_json(ss, pt, false);
        return ss.str();
}

cc @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)

To fix #4320 (closed)