websocket_client and json::value are oil and water on Windows

[DHowett-MSFT]

• websocket_client allows you to receive message bodies as streams of uint8_t or as std::strings, which are both utf-8 (also, per specification, the only valid encoding for JSON is utf-8)
• json::value::parse allows you to pass utility::string_t or utility::istream_t, which on Windows are typedef'd to wstring and basic_stream<wchar_t> (wistream) and therefore utf-16 (also, per specification, the only valid encoding for JSON is utf-8)

Without jumping through hoops, it is practically impossible to receive a message from a websocket client and parse it as JSON.

[garethsb]

There are std::istream (UTF-8) overloads on Windows. It would make sense to me to add std::string overloads as well.

https://github.com/microsoft/cpprestsdk/blob/b94bc32ff84e815ba44c567f6fe4af5f5f6b3048/Release/include/cpprest/json.h#L439-L459

[garethsb]

Resolved by #1350 (in 2.10.16)?