Test issue: @base syntax

Several of the tests appear to use invalid Turtle syntax for specifying the
base URI. Instead of using "@base:", they should use "@base" (note the extra
":" is removed).

For example, this file (as well as several others) has the problem:
https://dvcs.w3.org/hg/rdb2rdf-tests/file/92f2b4b0c9ca/1table0rows/r2rml.ttl

-David