PostgresqlExtDatabase with RealDictCursor causes KeyError on every select
Yeah, don't use that. If you want dicts, then use the `.dicts()` method. Peewee expects cursors to yield row tuples.

```python

q = MyModel.select().dicts()
for row_dict in q:
    # ...
```
Peewee version: 3.11.2
Postgresql version: 11
Db init:
```python
from playhouse.postgres_ext import PostgresqlExtDatabase
from psycopg2.extras import RealDictCursor

database = PostgresqlExtDatabase(
    None,
    register_hstore=False,
    autorollback = True,
    cursor_factory=RealDictCursor
)
```
Setting RealDictCursor cursor on PostgresqlExtDatabase database causes KeyError 0 on every select e.g.:
```
project-python |   File "/project/app/db/general.py", line 42, in get_all
project-python |     for row in cls.select(*fields).dicts():
project-python |   File "/root/.local/share/virtualenvs/project-nBADCc5g/lib/python3.6/site-packages/peewee.py", line 4224, in next
project-python |     self.cursor_wrapper.iterate()
project-python |   File "/root/.local/share/virtualenvs/project-nBADCc5g/lib/python3.6/site-packages/peewee.py", line 4143, in iterate
project-python |     result = self.process_row(row)
project-python |   File "/root/.local/share/virtualenvs/project-nBADCc5g/lib/python3.6/site-packages/peewee.py", line 7217, in process_row
project-python |     result[attr] = converters[i](row[i])
project-python | KeyError: 0
```
Moreover, any use of .create() method returns None without any warnings. I didn't find any information in the documentation or similar error, so I report it as a issue.

My current workaround is to use RealDictCursor only in context manager with raw SQL query, e.g.:
```python
with database.connection().cursor(cursor_factory=RealDictCursor) as cursor:
    cursor.execute("select * from table")
    print(cursor.fetchall())
```