Implement correct xsd:whitespace behaviour

[da1910]

Closes #1105
XSD spec requires specific treatment of whitespace for builtin types, whilst we cannot control the way any one of hundreds of soap servers return data, we can remove any whitespace that does not conform to the spec. This way we can be confident that the intent of the message is correctly captured.

I've implemented this with a new decorator, much like the @check_no_collections decorator currently used. This allows us to enforce the correct whitespace behaviour for each builtin type.

A summary of the desired behaviour is:

• String preserves whitespace,
• NormalizedString replaces \r\n\t with spaces
• Token and all other atomic types collapse multiple spaces and leading and trailing spaces

I have also added new asserts to check this behaviour for existing builtin tests, and added new tests for NormalizedString and Token which verify this behaviour. This breaks no existing tests, since we only provided valid inputs.