privatize method generate_doc
remove ugly print
fix test test_restore_in_dict

hector the dog is not registered so it should not be a SchemaDocument when it
come frem the mongo database. it must be a dict
remove type_field class variable from Document

as it is now in SchemaDocument
generate_doc does not return a dict anymore but modify self instead
fix test where to_json method is compare to a string

as in pyhton and json the order of the keys is not garantee,
we cannot compare to_json output directly with a hardcoded string.
I'ts better to transform both the hardcoded dtring and the to_json
output in json with the json library and compare the 2 resulting objects
Merge branch 'fix_test_json' into fix_restore_sd_in_dict