Support tab completion for upper character inputs in psql

Hi Hackers,

When using psql I found there's no tab completion for upper character inputs. It's really inconvenient sometimes so I
tryto fix this problem in the attached patch. 

Here is the examples to show what this patch can do.
Action:
1. connect the db using psql
2. input SQL command
3. enter TAB key(twice at the very first time)

Results:
[master]
postgres=# set a
all                      allow_system_table_mods  application_name         array_nulls
postgres=# set A

postgres=# set A

[patched]
postgres=# set a
all                      allow_system_table_mods  application_name         array_nulls
postgres=# set A
ALL                      ALLOW_SYSTEM_TABLE_MODS  APPLICATION_NAME         ARRAY_NULLS
postgres=# set A

Please take a check at this patch. Any comment is welcome.

Regards,
Tang