BUG #6181: concat_ws() incompatibility with MySQL

The following bug has been logged online:

Bug reference:      6181
Logged by:          Itagaki Takahiro
Email address:      itagaki.takahiro@gmail.com
PostgreSQL version: 9.1 RC1 (x86)
Operating system:   Windows 7 (64bit)
Description:        concat_ws() incompatibility with MySQL
Details:

For query concat_ws('/', NULL, 'X'),
PostgreSQL 9.1 returns '/X', but MySQL returns 'X'.

I think postgres should return 'X' because of compatibility.
Another reason is that both of the two queries below return 'X'.

- concat_ws('/', 'X', NULL)
- array_to_string(ARRAY[NULL, 'X'], '/')