fixed failing test HttpTest::testOriginalRequestExceptionIsPreserved

[kcasas]

/cc @zendesk/mintegrations

Description

• fixed a failing test

References

• Travis: https://travis-ci.org/zendesk/zendesk_api_client_php/builds/282557312?utm_source=github_status&utm_medium=notification

Risks

• [low] tests might still fail

[kcasas]

Closing this issue for now but this should be a reference if ever we update guzzle to 6.3