Subset not working with includes

[sbliven]

The subset validator accepts both single items and lists when used with primitives. However it does not validate single objects when included.

schema.yaml

primative_list: subset(str())
primative_single: subset(str())
complex_list: subset(include('person'))
complex_single: subset(include('person'))
---
person:
  first: str()
  last: str()

test.yaml

primative_list:
- foo
- bar
primative_single: foo
complex_list:
- first: joe
  last: dread
- first: sam
  last: scott

# Should validate but doesn't
complex_single:
  first: fred
  last: jones

Errors

complex_single.last: Required field missing
complex_single.first: Required field missing

This seems potentially related to #133 and #217, which also involve errors when combining validators.

[fclante]

Hey This is a big thing. Please prioritize this.