added a unit test for input/output file streams and fixed the problem it exposed (#3335)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@54246 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wfstream.h

@@ -76,7 +76,7 @@ public:
         Initializes a file stream in write-only mode using the file descriptor @e fp.
     */
     wxFFileOutputStream(const wxString& filename,
-                        const wxString& mode = "w+b");
+                        const wxString& mode = "wb");
     wxFFileOutputStream(wxFFile& file);
     wxFFileOutputStream(FILE* fp);
     //@}
@@ -242,7 +242,7 @@ public:
         Initializes a new file stream in read-write mode using the specified
         @e iofilename name.
     */
-    wxFFileStream(const wxString& iofileName);
+    wxFFileStream(const wxString& iofileName, const wxString& mode = "w+b");
 };