More characters are needed for guessing the format

FOAF schema file found at http://xmlns.com/foaf/spec/index.rdf has a long comments preface,
which doesn't fit in 255 characters. 600 does the trick though, so I changed the buffer size
and altered corresponding test

UPDATE: found another file, which has even longer preface. went up to 1024.
Merge pull request #130 from indeyets/rdfxml-guess

More characters are needed for guessing RDFXML format