BUG #4421: convert_to() should be immutable

The following bug has been logged online:

Bug reference:      4421
Logged by:
Email address:      andipeer@gmx.net
PostgreSQL version: 8.3.3
Operating system:   Linux
Description:        convert_to() should be immutable
Details:

The function convert_to(string text, dest_encoding name) is not allowed to
be used in a index expression, because it is not marked as "IMMUTABLE".
According to the documentation, a function is immutable if it does not
modify the database, and for the same arguments, it returns always the same
results. I think that all of these conditions can be applied to the
convert_to()-function, therefore it should be marked as "IMMUTABLE".