Specify attribute value syntax post-normalization

[nigelmegitt]

Following #170 and discussion in #185, specify all attribute value syntaxes as post-attribute value normalization form, i.e. replace <lwsp> with <nsp> where

<nsp>: #x20 after applying the normalization rules in [1]

[1] https://www.w3.org/TR/REC-xml/#AVNormalize

This also has the effect of reducing a possible ambiguity about whether leading and trailing white space is currently permitted or prohibited.

[nigelmegitt]

Since this has been actioned by the PR and the follow up issue #191 has been raised, closing.