Handle "#{}" as dynamic string

Before this commit, RDoc detects dynamic string
with only "#" inside string literal.
So "#" is handled as dynamic string.
This commit fixes the behavior.
When "#{" is contained in a string,
it is handled as dynamic string.

Author: aycabta

lib/rdoc/ruby_lex.rb

@@ -1225,8 +1225,8 @@ def identify_string(ltype, quoted = ltype, type = nil)
           break
         elsif @ltype != "'" && @ltype != "]" && @ltype != ":" and ch == "#"
           ch = getc
-          subtype = true
           if ch == "{" then
+            subtype = true
             str << ch << skip_inner_expression
             next
           else