With this code, based on the example in the readme, the base url is not used. If I remove .withContext(…), the base url is used.
JsonldRest.setBaseUrl('http://www.wikidata.org');
var entities = JsonldRest.collection('/entity').withContext({
"schema": "http://schema.org/",
"fullName": "schema:name"
});
entities.one('Q42').get().then(function(res) {
console.log(res);
});