Fixes #5715 - make sure the rest client doesn't get PhusionPassenger::Utils::TeeInput

Rest client 1.6.7 is not able to handle that, passing the string
instead. The behavior should stay the same.
Merge pull request #4086 from iNecas/rest-client-passenger

Fixes #5715 - make sure the rest client doesn't get PhusionPassenger::Utils::TeeInput