write_json fails when using basic_ptree with other string types

Good morning,

I was working on a project, I would like to use basic_ptree with strings of non standard type (see T1.cpp attached toy example).

While read_json works smootly, write_json fails (see compiler output T1.error.txt).

I checked the code and, apparently, an easy fix could enable this feature (see write.hpp which I revised for your convenience, original lines of code are commented, those inserted are marked with an eol comment "suggested").

Thank you for your time,
Marco.

[write.hpp.txt](https://github.com/boostorg/property_tree/files/4908338/write.hpp.txt)
[T1.cpp.txt](https://github.com/boostorg/property_tree/files/4908339/T1.cpp.txt)
[T1.error.txt](https://github.com/boostorg/property_tree/files/4908340/T1.error.txt)

Start by compiling the attached T1.cpp with the current write.hpp and compare the failure with the provided T1.error.txt. Trace the write_json path for basic_ptree and non-standard string types, then verify the reported case serializes successfully without breaking the existing read_json behavior.

Written by the indexing model from the issue text.