Allow referencing to the same document

[tomschr]

According to the XInclude spec, the href attribute is optional:

The href attribute is optional; the absence of this attribute is the same as specifying href="", that is, the reference is to the same document.

I think it does make sense to allow referencing and including parts from the same document.

Some ideas:

• When there is no href attribute (or href="), the spec requires to have either an xpointer or fragid attribute.
• ATM it's enough to support fragid only.
• I think, we should also check if set-xml-id is there. We don't want the same IDs in the result document.
• If there is no such an attribute, should dbxincluder print a warning? Abort?

[Vogtinator]

I think, we should also check if set-xml-id is there. We don't want the same IDs in the result document.

There are documents that have no xml:id anywhere, so set-xml-id isn't required. Also, the spec says that XInclude can generate invalid documents, so not allowing that would be an error.

[Vogtinator]

Implemented.