Fix #update bug in DocumentMetadataDecorator

The extra attributes hash was completely replaced
on update with whatever was in the given hash.

This required the caller to provide all the
key/value pairs for the extra attributes or have
them wiped out. This was inconsistent with the
update behaviour of the other fields.
Merge pull request #175 from alphagov/fix-document-metadata-decorator

Fix #update bug in DocumentMetadataDecorator