fixed access to coerceIntegerPrimaryKey in Modal class functions

I found this bug when loading an array of records with their primary key is string, but those records cannot be put into identity maps. This PR will fix this issue.

A good question is why making `coerceIntegerPrimaryKey` a prototype property? Since most of Model settings such as `primaryKey` is defined in constructor, it's really weird that `coerceIntegerPrimaryKey` doesn't follow this convention.