Handling of base URI

[redaktor]

trafficstars

There might be some issues with the handling of base URI.

• if n3 has @base or rdf/xml has xml:base : Shouldn't it supersede any previous given base URI ?
• when serializing from n3 to rdf/xml base URI seems to get lost : Shouldn't it become the xml:base attribute in the <rdf:RDF> ? (same vice-versa)
• when working with local files: Shouldn't it be a MUST to specify a base URI as fallback instead of falling back to the local path ?, seeAlso: the 'beware of this' comments by Sir timbl in sparqlUpdate.js

[ManuelTS]

Might be relevant to #245 and #251