Ensure that Editor.proto.serialize returns a copy of each edit session

spec/app/editor-spec.coffee

@@ -40,6 +40,9 @@ describe "Editor", ->
       expect(newEditor.buffer).toBe editor.buffer
       expect(newEditor.getCursorScreenPosition()).toEqual editor.getCursorScreenPosition()
 
+      expect(newEditor.editSessions[0]).toEqual(editor.editSessions[0])
+      expect(newEditor.editSessions[0]).not.toBe(editor.editSessions[0])
+
   describe "text rendering", ->
     it "creates a line element for each line in the buffer with the html-escaped text of the line", ->
       expect(editor.lines.find('.line').length).toEqual(buffer.numLines())

src/app/editor.coffee

@@ -38,6 +38,7 @@ class Editor extends View
   isFocused: false
   softTabs: true
   tabText: '  '
+  editSessions: null
 
   @deserialize: (viewState) ->
     new Editor(viewState)
@@ -64,7 +65,10 @@ class Editor extends View
 
   serialize: ->
     @saveCurrentEditSession()
-    { viewClass: "Editor", @editSessions, @activeEditSessionIndex, @isFocused }
+    { viewClass: "Editor", editSessions: @serializeEditSessions(), @activeEditSessionIndex, @isFocused }
+
+  serializeEditSessions: ->
+    @editSessions.map (session) -> _.clone(session)
 
   copy: ->
     Editor.deserialize(@serialize())
@@ -512,7 +516,7 @@ class Editor extends View
     return super if keepData
     @unsubscribeFromBuffer()
     rootView = @rootView()
-    if @pane() then @pane.remove() else super
+    if @pane() then @pane().remove() else super
     rootView?.focus()
 
   unsubscribeFromBuffer: ->