(Polished initial patch by Alexey Botchkov)
Make the code handle DEFAULT values of any datatype

- Make Json_table_column::On_response::m_default be Item*, not LEX_STRING.
- Change the parser to use string literal non-terminals for producing
  the DEFAULT value
-- Also, stop updating json_table->m_text_literal_cs for the DEFAULT
   value literals as it is not used.