fixed deleteBackwards under iOS 7 for list prefixes
fixed deleteBackwards under iOS 7 for list prefixes

liulinjian/DTRichTextEditor,fishermenlabs/DTRichTextEditor,programming086/DTRichTextEditor,Cocoanetics/DTRichTextEditor,fishermenlabs/DTRichTextEditor,programming086/DTRichTextEditor,liulinjian/DTRichTextEditor