Make django_orm models use TEXT instead of VARCHAR.

Reviewed in http://codereview.appspot.com/5165047/
diff --git a/oauth2client/django_orm.py b/oauth2client/django_orm.py
index 5e511db..d3642f7 100644
--- a/oauth2client/django_orm.py
+++ b/oauth2client/django_orm.py
@@ -31,8 +31,8 @@
 
   __metaclass__ = models.SubfieldBase
 
-  def db_type(self, connection=None):
-    return 'VARCHAR'
+  def get_internal_type(self):
+    return "TextField"
 
   def to_python(self, value):
     if not value:
@@ -41,7 +41,7 @@
       return value
     return pickle.loads(base64.b64decode(value))
 
-  def get_db_prep_value(self, value):
+  def get_db_prep_value(self, value, connection, prepared=False):
     return base64.b64encode(pickle.dumps(value))
 
 
@@ -49,8 +49,8 @@
 
   __metaclass__ = models.SubfieldBase
 
-  def db_type(self, connection=None):
-    return 'VARCHAR'
+  def get_internal_type(self):
+    return "TextField"
 
   def to_python(self, value):
     if value is None:
@@ -59,7 +59,7 @@
       return value
     return pickle.loads(base64.b64decode(value))
 
-  def get_db_prep_value(self, value):
+  def get_db_prep_value(self, value, connection, prepared=False):
     return base64.b64encode(pickle.dumps(value))