fix: Use model.Type() as cache key instead of model itself

[max-stytch]

Fix for #61

Type values are comparable, such as with the == operator, so they can be used as map keys. Two Type values are equal if they represent identical types.

From https://pkg.go.dev/reflect#Type

[coveralls]

coverage: 98.8% (+0.01%) from 98.79% when pulling 766c9a4 on max-stytch:max/fix-mem-leak into 0eac140 on blockloop:master.

[blockloop]

This looks great. I was actually just writing a fix for it. Thank you for this PR.

[blockloop]

I just cut v2.2.0 with this patch. Thanks again

[max-stytch]

Thank you!