catch better error for duplicate values

[jvanasco]

If a schema has a scalar field (such as text or email) and multiple items are submitted, Formencode will raise a Python exception instead of catching the python exception and making the field invalid.

for example, if formencode is leveraged in a website and multiple values are submitted as GET query arguments such as http://example.com?foo=bar&foo=biz, formencode will raise AttributeError: 'list' object has no attribute 'strip'.

I suggest formencode catch these exceptions and mark the field as invalid.