Bugfix initializing DocBin with attributes

[svlandeg]

Description

We called intify_attrs() with a list instead of with a dictionary when creating a new DocBin. I created a small method intify_attr() that takes just one string argument to apply it quickly on lists.

I also added the example from the docs to the unit tests, and fixed a typo in the example.

Fixes #4367

Types of change

bug fix + documentation fix

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[honnibal]

Bloody Python 2.7 :(. I guess we need a spacy.compat function gzip_compress: https://docs.python.org/2/library/gzip.html

[honnibal]

Ah, nice solution.