BUG #6718: Cannot delete, create or check existence of extension

The following bug has been logged on the website:

Bug reference: 6718
Logged by: Gary Haran
Email address: gary(dot)haran(at)gmail(dot)com
PostgreSQL version: 9.1.4
Operating system: OS X
Description:

development=# select version();
version

-----------------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.4 on x86_64-apple-darwin11.4.0, compiled by Apple clang
version 3.1 (tags/Apple/clang-318.0.61) (based on LLVM 3.1svn), 64-bit
(1 row)

development=# create extension hstore;
ERROR: type "hstore" already exists
development=# drop extension hstore;
ERROR: extension "hstore" does not exist
development=# CREATE EXTENSION IF NOT EXISTS hstore;
ERROR: type "hstore" already exists