show schema.collate in explain(verbose on)

Hi, 

The comment of get_collation_name wrote: 

   * NOTE: since collation name is not unique, be wary of code that uses this
   * for anything except preparing error messages.

In file explain.c, the function get_collation_name() is called, but the schema 
name won't be outputted.

Since the schema name of relation/function name will be outputted in 
explain's result when verbose(on) is specified.  I plan to output the schema 
name of collation as well as relation/function.

The patch attached, any thought?

Regards
Shenhao Wang