title: Extension exists but uuid_generate_v4 fails
body: <p>At amazon ec2 RDS Postgresql:</p>
  
  <pre><code>=&gt; SHOW rds.extensions;
  
  rds.extensions                                                                                                                                 
  --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   btree_gin,btree_gist,chkpass,citext,cube,dblink,dict_int,dict_xsyn,earthdistance,fuzzystrmatch,hstore,intagg,intarray,isn,ltree,pgcrypto,pgrowlocks,pg_trgm,plperl,plpgsql,pltcl,postgis,postgis_tiger_geocoder,postgis_topology,sslinfo,tablefunc,tsearch2,unaccent,uuid-ossp
  (1 row)
  </code></pre>
  
  <p>As you can see, <code>uuid-ossp</code> extension does exist. However, when I'm calling the function for generation <code>uuid_v4</code>, it fails:</p>
  
  <pre><code>CREATE TABLE my_table (
      id uuid DEFAULT uuid_generate_v4() NOT NULL,
      name character varying(32) NOT NULL,
  
  );
  </code></pre>
  
  <p>What's wrong with this?</p>