db.atomic() does not set transaction_type
Thanks for the report, pushed 2.8.8.
Regression after updating from 2.8.5 to 2.8.7:

```
  File "/Users/jan/projects/lexi/importer/src/main/python/lexi/models/fts.py", line 160, in index
    with database.atomic():
  File "/Users/jan/projects/lexi/importer/venv-python2/lib/python2.7/site-packages/peewee.py", line 4356, in __enter__
    return self.context_manager.__enter__()
  File "/Users/jan/projects/lexi/importer/venv-python2/lib/python2.7/site-packages/peewee.py", line 4381, in __enter__
    if self.db.transaction_depth() == 0: self._begin()
  File "/Users/jan/projects/lexi/importer/venv-python2/lib/python2.7/site-packages/playhouse/apsw_ext.py", line 46, in _begin
    self.db.begin(self.lock_type)
  File "/Users/jan/projects/lexi/importer/venv-python2/lib/python2.7/site-packages/playhouse/apsw_ext.py", line 127, in begin
    self.get_cursor().execute('begin %s;' % lock_type)
  File "src/cursor.c", line 1019, in APSWCursor_execute.sqlite3_prepare
  File "src/statementcache.c", line 386, in sqlite3_prepare
SQLError: SQLError: near "None": syntax error
```

class Database:
```python
def atomic(self, transaction_type=None):
        return _atomic(self, transaction_type)
```

```python
class _atomic(_callable_context_manager):
   def __enter__(self):
        if self.db.transaction_depth() == 0:
            self.context_manager = self.db.transaction(self.transaction_type)
   ...
```

class APSWDatabase:
```python
def begin(self, lock_type='deferred'):
        self.get_cursor().execute('begin %s;' % lock_type)
                                                ^^^^^^^^^ = None
```
