fix issue #2452 - displacy arrow direction is always forward (#2506) (closes #2452)
fix issue #2452 - displacy arrow direction is always forward (#2506) (closes #2452)

<!--- Provide a general summary of your changes in the title. -->
Referring #2452, fixing displacy arrow directions to match the input. 

## Description
The fix is simply replacing `direction is 'left'` with `direction == 'left'` to include the case `direction` is a `str` and not a `unicode`.

### Types of change
bug fix

## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [ ] I have submitted the spaCy Contributor Agreement.
- [ ] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.

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