Don't coerce Model ids like "15" to integers

If a model's `id` attribute is a  string, but looks like an integer, it is [coerced to an integer] (https://github.com/batmanjs/batman/blob/master/src/model/model.coffee#L295) by Batman.Model. However, when a hasMany is loaded, its [raw JSON value](https://github.com/batmanjs/batman/blob/master/src/model/associations/has_many_association.coffee#L51) (a string in this case) is used to find if it was already loaded. This causes a double-load, because all the ids on the client have been converted to integers already -- the string doesn't find a match!

I added a [test case with integer-like string ids](https://github.com/batmanjs/batman/pull/866/files#diff-c2fd66577ff1584198b81349b0f3f241R562). ~~which failed until I [fetched and used encoder.decode](https://github.com/batmanjs/batman/pull/866/files#diff-6dd1788cb8af56da2d86145cdc6484d6L49) when decoding a hasMany.~~

To get around this, I removed the coercion in from the `@wrapAccessor` in Batman.model.