Prior to this commit, the `GraphQlTester#executeAndVerify` method would check that an `"$.errors"` JSON path exists within the response and contains no value. This should fail with valid GraphQL responses, as the spec says that a response with no error should not contain the errors map at all: http://spec.graphql.org/draft/#sec-Errors This commit changes the assertion and instead checks for the absence of the errors map in the response. Fixes gh-318