Fix expect on content-type header test

Author: fbaiodias

test/request-api.spec.js

@@ -55,7 +55,7 @@ describe('ipfsAPI request tests', () => {
       }).listen(6001, () => {
         ipfsAPI('/ip4/127.0.0.1/tcp/6001')
           .config.replace('test/r-config.json', (err) => {
-            expect(err).to.be.equal(null)
+            expect(err).to.not.exist
             server.close(done)
           })
       })