Fix fmt::localtime formatting not working in wide-char string contexts

[yNeo]

Issue was caused by incorrectly using an explicit type for parse_context parameter instead of a templated type, which meant that the parse function would only work in regular string contexts.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks. LGTM but could you add a test case in https://github.com/fmtlib/fmt/blob/master/test/chrono-test.cc?

[vitaut]

Added a test and merged in c5979d5.