Add specs for Document#fetch
Add specs for Document#flatten
Add specs for Document#has_key?
Add specs for Document#has_value?
Add specs for Document#include?
Add specs for Document#invert
Add specs for Document#keep_if
Add specs for Document#key
Add specs for Document#key?
Add specs for Document#length
Add specs for Document#size
Add specs for Document#member?
Add specs for Document#value?