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.

spacy/displacy/render.py

@@ -136,7 +136,7 @@ class DependencyRenderer(object):
         end (int): X-coordinate of arrow end point.
         RETURNS (unicode): Definition of the arrow head path ('d' attribute).
         """
-        if direction is 'left':
+        if direction == 'left':
             pos1, pos2, pos3 = (x, x-self.arrow_width+2, x+self.arrow_width-2)
         else:
             pos1, pos2, pos3 = (end, end+self.arrow_width-2,