allow Regexp objects for strings in the schema

allow Regexp objects for strings in the schema

The 'pattern' property has type string and format regex, so
it makes sense to allow Regexp objects to be used for it.

While check_type() doesn't know the format, Regexp objects
can be treated like strings anyway, including compared via
'eq' or matched via '=~', so we allow strings to generally
come from a Regexp object.