Stop dropping \ after \

In string or regexp literal,
\ is a escape character and continuing \ is dropped.
This Pull Request stop the dropping.

Author: aycabta

lib/rdoc/ruby_lex.rb

@@ -1235,7 +1235,7 @@ def identify_string(ltype, quoted = ltype, type = nil)
         elsif ch == '\\'
           if %w[' /].include? @ltype then
             case ch = getc
-            when "\\", "\n", "'"
+            when "\n", "'"
             when @ltype
               str << ch
             else