Unicode handling broken for NextContinuationToken

[Darrell Bishop]

Unicode handling broken for NextContinuationToken in servicing container listings.

This fixes it:

```
diff -ru swift3.orig/swift3/controllers/bucket.py swift3/swift3/controllers/bucket.py
--- swift3.orig/swift3/controllers/bucket.py 2018-03-21 21:23:40.000000000 +0000
+++ swift3/swift3/controllers/bucket.py 2018-05-01 20:08:03.589958449 +0000
@@ -156,10 +156,10 @@
             if is_truncated:
                 if 'name' in objects[-1]:
                     SubElement(elem, 'NextContinuationToken').text = \
- b64encode(objects[-1]['name'])
+ b64encode(objects[-1]['name'].encode('utf8'))
                 if 'subdir' in objects[-1]:
                     SubElement(elem, 'NextContinuationToken').text = \
- b64encode(objects[-1]['subdir'])
+ b64encode(objects[-1]['subdir'].encode('utf8'))
             if 'continuation-token' in req.params:
                 SubElement(elem, 'ContinuationToken').text = \
                     req.params['continuation-token']
```