#181 Update schema on PloneArticle crashes Zope

[Anton Stonor]

If you try to update the schema (using archetypes tool)
of PloneArticle or a PloneArticle-based content type,
Zope crashes.

The offending line is in the PloneArticle class:

schema['text'].allowable_content_types =
ComputedAttribute(getPloneArticleAllowableContentTypes, 1)

The problem is, that the toString method on
Archetypes.Field.Field does not expect data types like
ComputedAttributes on field properties.

Observed with PloneArticle 3.19.