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;
 }
 

tests/controls/richtextctrltest.cpp

@@ -34,6 +34,7 @@ public:
 
 private:
     CPPUNIT_TEST_SUITE( RichTextCtrlTestCase );
+        CPPUNIT_TEST( IsModified );
         WXUISIM_TEST( CharacterEvent );
         WXUISIM_TEST( DeleteEvent );
         WXUISIM_TEST( ReturnEvent );
@@ -64,6 +65,7 @@ private:
         CPPUNIT_TEST( Table );
     CPPUNIT_TEST_SUITE_END();
 
+    void IsModified();
     void CharacterEvent();
     void DeleteEvent();
     void ReturnEvent();
@@ -115,6 +117,13 @@ void RichTextCtrlTestCase::tearDown()
     wxDELETE(m_rich);
 }
 
+void RichTextCtrlTestCase::IsModified()
+{
+    CPPUNIT_ASSERT_EQUAL( false, m_rich->IsModified() );
+    m_rich->WriteText("abcdef");
+    CPPUNIT_ASSERT_EQUAL( true, m_rich->IsModified() );
+}
+
 void RichTextCtrlTestCase::CharacterEvent()
 {
 #if wxUSE_UIACTIONSIMULATOR