Define a more restrictive type for Issuer attributes

[cdanger]

The attribute Issuer is defined as xs:string in Attribute, AttributeDesignator, AttributeAssignment, AttributeAssignmentExpression, and MissingAttributeDetail types. Like in #42, xs:string allows any XML text to be used as Issuer, including an empty string "" or a string starting/ending with whitespace(s) or containing a sequence of multiple whitespaces, or a string filled with whitespaces.
More appropriate types like xs:token or a new non-empty non-whitespace string type, i.e. using pattern [^\s]+, could avoid that.