Fixed default value getter for "owners" field not from the plone default...

This is a follow up from PR #27.

Because the equality check for fields in [`zope.schema`](https://github.com/zopefoundation/zope.schema/blob/978f020ddc8b8be57cddca57e7c95df42411d7cf/src/zope/schema/_bootstrapfields.py#L187) is insufficient, some special check for the `IOwnership.creators` fix from the PR #27 is needed.

Because a different tuple field with the name `creators` (for example in [opengever.core](https://github.com/4teamwork/opengever.core/blob/master/opengever/base/behaviors/creator.py#L19) is handled as the `IOwnership.creators` field, because the `field.interface` is not compared in the equality check. See the following debugging output:
```
(Pdb) field.interface
<InterfaceClass opengever.base.behaviors.creator.ICreator>
(Pdb) field
<zope.schema._field.Tuple object at 0x106f9b590>
(Pdb) IOwnership['creators']
<zope.schema._field.Tuple object at 0x10698e150>
(Pdb) IOwnership['creators'].interface
<InterfaceClass plone.app.dexterity.behaviors.metadata.IOwnership>
(Pdb) field == IOwnership['creators']
True
```
@jone, @deiferni