Nope, its more that Passenger implements a rack interface whereby its input object features a rewind() object, whereas Rest Client is expecting a StringIO.  While they implement almost the same interface, they don't quite. So this is just translating a StringIO seek(0) into a rewind() method.

hrmmm  I'm not entirely sure how to test this, as passenger isn't even loaded in development/test.   Adding it as a Gem requirment doesn't even seem to make it available.