Fix compatibility with Rack 1.4.3 in 1.9 mode (TORQUE-993)

Rack added an each method to their BodyProxy which means every body
now responds to .each, even if our underlying body is a String in 1.9
mode where Strings don't have an each method.

Technically the Rack spec says our body should not be a String
instance, but moving away from that is a bigger change than I want to
make right now.