'possible loss of data' warning when using wide-character wtime_input_facet

Compilers emit a 'possible loss of data' warning when using `wtime_input_facet::get()`.
E.g. Visual C++ emits "_warning C4244: 'argument': conversion from 'const wchar_t' to 'const _Elem', possible loss of data_" with the following code:

wstring s;
// in.imbue({ locale::classic(), new wtime_input_facet{L"%m-%e-%Y %H:%M:%S"} });
wistringstream in{ s };
ptime pt;
in >> pt;

This is because `wtime_input_facet::get()` aka `time_input_facet>::get()` checks for errors in regard to special formatting characters, and the error handling code path effectively converts the wide-character input sequence to a narrow-character std::string, `date_time::convert_string_type` being the culprit in date_time library land.

The call and warning message stack look like this:

> 26: date_time::convert_string_type(const wstring& inp_str)
|-> 1260: date_time::wtime_input_face::check_special_value(istreambuf_iterator& sitr, istreambuf_iterator& stream_end, ptime& tt, wchar_t c)
|-> 1125: date_time::wtime_input_face::get(istreambuf_iterator& sitr, istreambuf_iterator& stream_end, ios_base& ios_arg, ptime& t, wstring& tz_str, bool time_is_local)
|-> 936: date_time::wtime_input_face::get(istreambuf_iterator& sitr, istreambuf_iterator& stream_end, ios_base& ios_arg, ptime& t)
|-> 80: posix_time::operator>>(wistream& is, ptime& pt)

I am currently uncertain how to fix this properly, but it is definitely a PITA because of the lengthy warning message stack.