Fix string detection in Js::Returns.

Because the EsprimaAdapter gives a string value to regex literals,
which is different from nil received from Esprima itself, we'll
have to check first for regex and after that for string.

lib/jsduck/js/returns.rb

@@ -79,10 +79,10 @@ module JsDuck
           :this
         elsif boolean?(ast)
           "Boolean"
-        elsif string?(ast)
-          "String"
         elsif regexp?(ast)
           "RegExp"
+        elsif string?(ast)
+          "String"
         else
           :other
         end