Fix "Cannot call method 'toLowerCase' of undefined" error

sequelize/lib/dao.js:139
ap(function(d) { return d.toLowerCase() }).indexOf(values[attrName].toLowerCas
                                                                    ^
TypeError: Cannot call method 'toLowerCase' of undefined
Merge pull request #988 from KevinMartin/patch-1

Fix "Cannot call method 'toLowerCase' of undefined" error