Fix wxStringOutputStream in wxUSE_UNICODE_UTF8 build.

Fix wxStringOutputStream in wxUSE_UNICODE_UTF8 build.

For some reason the conversion of the bytes written to this stream to Unicode
was only done in wxUSE_UNICODE_WCHAR build but not in wxUSE_UNICODE_UTF8 one.
Do it in any wxUSE_UNICODE build now.

This allows to use wxStringOutputStream under Unix again, in particular it
fixes an assert in samples/html/zip when trying to load the raw contents of a
ZIP file in wxHtmlWindow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67968 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775