Don't add default extension in wxDocument::SaveAs().

Don't add default extension in wxDocument::SaveAs().

wxFileSelector() takes care of the default extension itself and if it returned
a file name without one, it means that the user really wants to create a file
without an extension. Adding the default extension here is at best useless and
at worst actively dangerous because it may silently overwrite an existing file
(although this bug is due to a different problem, see #11256 for a scenario in
which this can happen).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62350 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775