fix migrate='safe'

I get this error when I use the option `migrate='safe'`
```
[err] Server Error (500)
[err] TypeError: Object.keys called on non-object
    at Function.keys (native)
    at Object.utils.buildSelectStatement (/home/proyectobiota/node_modules/sails-postgresql/lib/utils.js:338:10)
    at Query.find (/home/proyectobiota/node_modules/sails-postgresql/lib/query.js:41:23)
    at __FIND__ (/home/proyectobiota/node_modules/sails-postgresql/lib/adapter.js:397:30)
    at after (/home/proyectobiota/node_modules/sails-postgresql/lib/adapter.js:534:7)
    at /home/proyectobiota/node_modules/sails-postgresql/lib/adapter.js:520:7
    at /home/proyectobiota/node_modules/pg/lib/pool.js:55:9
    at dispense (/home/proyectobiota/node_modules/pg/node_modules/generic-pool/lib/generic-pool.js:247:16)
    at Object.me.acquire (/home/proyectobiota/node_modules/pg/node_modules/generic-pool/lib/generic-pool.js:316:5)
    at Object.pool.connect (/home/proyectobiota/node_modules/pg/lib/pool.js:53:12)
```
I figure out that in master branch, the function `registerCollection` always calls `describe`, so I did that here, then the bug was fixed.