title: How get a peewee DateField as string in a query returning tuples
body: <p>I'm attempting to make a query with  a DateField in peewee and returning the result as Tuples. I'm using SQLite as database,  the query works ok but in the DateField is returning the DateTime object instead  the date as string.</p>
  <p>This is the code:</p>
  <pre><code>import peewee as pw
  
  db = pw.SqliteDatabase(':memory:')
  
  
  class Person(pw.Model):
      name = pw.CharField()
      birthday = pw.DateField(formats=['%Y-%m-%d'])
  
      class Meta:
          database = db
  
  db.create_tables([Person])
  
  alice = Person(name = 'Alice', birthday = '1995-1-21')
  robert = Person(name = 'Robert', birthday = '1978-1-22')
  john = Person(name = 'John', birthday = '1995-1-20')
  mary = Person(name = 'Mary', birthday = '1989-1-18')
  emily = Person(name = 'Emily', birthday = '2003-12-28')
  alice.save()
  robert.save()
  john.save()
  mary.save()
  emily.save()
  
  query = Person.select(Person.name,
                        Person.birthday).tuples()
  
  
  for rows in query:
      print(rows)
  
  </code></pre>
  <p>So what I get is this:</p>
  <pre><code>('Alice', datetime.date(1995, 1, 21))
  ('Robert', datetime.date(1978, 1, 22))
  ('John', datetime.date(1995, 1, 20))
  ('Mary', datetime.date(1989, 1, 18))
  ('Emily', datetime.date(2003, 12, 28))
  [Finished in 85ms]
  </code></pre>
  <p>I would get  the result of query like:</p>
  <pre><code>('Alice', '1995-1-21')
  ('Robert', '1978-1-22')
  ('John', '1995-1-20')
  ('Mary', '1989-1-18')
  ('Emily', '2003-12-28')
  
  </code></pre>
  <p>There is a way I could achive that?</p>
  <p>I tried converting the list of tuples to list of list:</p>
  <pre><code>qrylist = [list(items) for items in query] 
  </code></pre>
  <p>but the result is the same:</p>
  <pre><code>['Alice', datetime.date(1995, 1, 21)]
  ['Robert', datetime.date(1978, 1, 22)]
  ['John', datetime.date(1995, 1, 20)]
  ['Mary', datetime.date(1989, 1, 18)]
  ['Emily', datetime.date(2003, 12, 28)]
  </code></pre>