Adjust specs to match new graphql-js behavior

We switched from throwing to returning an error for documents with
multiple operations which don't specify an operation name to execute.

See: graphql/graphql-js@508aa3a2b6

Author: wincent

src/__tests__/http-test.js

@@ -263,7 +263,7 @@ describe('test harness', () => {
           }),
         );
 
-        expect(response.status).to.equal(400);
+        expect(response.status).to.equal(200);
         expect(JSON.parse(response.text)).to.deep.equal({
           errors: [
             {