Use em_delete in key_delete

When the editing mode is emacs, use `em_delete` in `key_delete`. We need
to add a condition though to `em_delete`, because it implements both
`delete-char` and `end-of-file`. We only want the `end-of-file` behavior
is the key is really Ctrl-D.

This matches the behavior of the <Del> key with readline, i.e. deleting
the next character if there is one, but not moving the cursor, while not
finishing the editing if there are no characters.

lib/reline/line_editor.rb

@@ -1944,8 +1944,10 @@ def finish
   end
 
   private def key_delete(key)
-    if @config.editing_mode_is?(:vi_insert, :emacs)
+    if @config.editing_mode_is?(:vi_insert)
       ed_delete_next_char(key)
+    elsif @config.editing_mode_is?(:emacs)
+      em_delete(key)
     end
   end
 
@@ -2651,7 +2653,7 @@ def finish
   alias_method :kill_whole_line, :em_kill_line
 
   private def em_delete(key)
-    if @line.empty? and (not @is_multiline or @buffer_of_lines.size == 1)
+    if @line.empty? and (not @is_multiline or @buffer_of_lines.size == 1) and key == "\C-d".ord
       @line = nil
       if @buffer_of_lines.size > 1
         scroll_down(@highest_in_all - @first_line_started_from)

test/reline/test_key_actor_emacs.rb

@@ -473,6 +473,17 @@ def test_key_delete_does_not_end_editing
     refute(@line_editor.finished?)
   end
 
+  def test_key_delete_preserves_cursor
+    input_keys('abc')
+    input_keys("\C-b", false)
+    assert_cursor(2)
+    assert_cursor_max(3)
+    @line_editor.input_key(Reline::Key.new(:key_delete, :key_delete, false))
+    assert_cursor(2)
+    assert_cursor_max(2)
+    assert_line('ab')
+  end
+
   def test_em_next_word
     assert_byte_pointer_size('')
     assert_cursor(0)