DictionaryProperty __init__ should take argument which specified the valid types for the value of the dictionary items

[rpiazza]

There are normative statements in the spec about the value of dictionary items for properties that are defined to be of the dictionary type (e.g., email-message.additional_header_fields). They are not checked in the clean method.

So far, I have added the valid_types argument to the DictionaryProperty init method which is saved on the specifics member of the class instance.

There are five possible values:

• None - defaults to 'string'
• 'string'
• 'integer'
• 'string_list'
• [ string', 'integer']

Things to do:

• don't allow 'string_list' as one of the values when a list is given for valid_types
• in clean, check that the value of the dictionary items is the correct type as noted in specifics.
• some Tests to check for both the correct use of the DictionaryProperty definitions and instantiations