content_type is a bit special, because CMF's PortalFolderBase also have
a content_type method, so this was confusing ERP5Type dynamic properties
system. When the property was never set, the default value was the
method acquired from PortalFolderBase.

Fixes this by defining conten_type class attribute where needed.

This also fixes an old expected failure test, it's now possible to use
edit(content_type=something) in document without content type property
set. It works now, but thanks to what seems to be outdated code in
_valid_property_id from product/ERP5Type/patches/PropertyManager.py,
because we have stopped setting all properties to None in Base years
ago. For now, this works, but because of an outdated condition in
_valid_property_id. We may revisit later in the future.