Eeschema: fix sheet file name case sensitivity test.

Fix overzealous test if the new file name is the same as an existing
file name.

Fix a potential comparison bug due to missing file extension.

eeschema/sch_screen.cpp

@@ -1371,10 +1371,10 @@ bool SCH_SCREENS::CanCauseCaseSensitivityIssue( const wxString& aSchematicFileNa
         if( lhs.GetPath() != rhs.GetPath() )
             continue;
 
-        lhsLower = lhs.GetName().Lower();
-        rhsLower = rhs.GetName().Lower();
+        lhsLower = lhs.GetFullName().Lower();
+        rhsLower = rhs.GetFullName().Lower();
 
-        if( lhsLower == rhsLower )
+        if( lhsLower == rhsLower && lhs.GetFullName() != rhs.GetFullName() )
             return true;
     }