title: sqlite3 InterfaceError after upgrading to Peewee 2.6
body: <p>After upgrading to Peewee 2.6.3 from v2.4.7, the following error pops up when iterating on results from a select query.</p>
  
  <p><strong>EDIT</strong>: it seems to be a bug related to the use of the create_or_get method.</p>
  
  <pre><code> File "./script.py", line 137, in load_data
      for name,country in rows:
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/peewee.py", line 1957, in next
      obj = self.iterate()
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/peewee.py", line 1934, in iterate
      row = self.cursor.fetchone()
  sqlite3.InterfaceError: Cursor needed to be reset because of commit/rollback and can no longer be fetched from. 
  </code></pre>
  
  <p>Here is the offending code:</p>
  
  <pre><code>for i in range(1, nb_rows // CHUNK_SIZE + 2):                                                                                                                                                                                                                                                                                                     
      rows = (Source.select(Source.name,Source.country)                                                                                                                       
             .where(Source.status == 'new').paginate(i, CHUNK_SIZE).tuples())                                                                                                     
  
      for name,country in rows:                                                                                                                                                                                                                                                                                                               
          person, created = Person.create_or_get(name = name, new = True)                                                                                                        
  
          if country:                                                                                                                                                                   
              country, created = Countries.create_or_get(name = country)                                                                                                                         
              Person_Country(person_id = person.id, country_id = country.id).save()   
  </code></pre>
  
  <p>Can you help me identify what the problem is and how to fix it?</p>
  
  <p>Note: using Sqlite3 3.8.9                                                                                                        </p>