Fix percent literal with the same sign of original

In identify_string method,
percent literal is detected by literal type and quote sign.
So %r/regexp/ is treated as /regexp/,
because literal type is regexp and quote sign is /, it's the same.
This commit fix it
by checking "type" argument what is given only when percent literal.

Author: aycabta

lib/rdoc/ruby_lex.rb

@@ -1306,7 +1306,7 @@ def identify_string(ltype, quoted = ltype, type = nil)
     @ltype = ltype
     @quoted = quoted
 
-    str = if ltype == quoted and %w[" ' /].include? ltype then
+    str = if ltype == quoted and %w[" ' / `].include? ltype and type.nil? then
             ltype.dup
           else
             "%#{type}#{PERCENT_PAREN_REV[quoted]||quoted}"