String type must be strict.

It makes sense to coerce from more general -> more specific data types.
eg. string -> int, etc. But coercing the other way is problematic
in the case of chaining because the more general type will swallow the
specific type. eg. Setting `false` on type: [‘string’, ‘boolean’] will 
coerce the boolean to a string, and will never allow the value to be
a boolean.