PR# 10079 Bug in EV_RICH_TEXT.select_region

I think that there is a bug in EV_RICH_TEXT.select_region. When the `start_pos' and `end_pos' are the same, e.g.:

    my_rich_text.select_region (2, 2)

EiffelVision actually selects the character between positions 2 and 3. I don't think that I violate any precondition when `start_pos' = `end_pos', but in that case I don't see how both postconditions "has_selection" and "selection_set" can be true. It's either one or the other (provided that "has_selection" really means that something is selected in the text).