Fix initial value returned from wxRichTextCtrl::IsModified()

A just created control shouldn't be considered modified, so reset its
modified state in Create().

And add a unit test checking that IsModified() really behaves as
expected.

Closes https://github.com/wxWidgets/wxWidgets/pull/1652

src/richtext/richtextctrl.cpp

@@ -343,7 +343,7 @@ bool wxRichTextCtrl::Create( wxWindow* parent, wxWindowID id, const wxString& va
 #if wxUSE_DRAG_AND_DROP
     SetDropTarget(new wxRichTextDropTarget(this));
 #endif
-
+    SetModified( false );
     return true;
 }