The HttpRequest trait doesn't make sense, on further reflection. Rather,
just modify the method signature on the requester. The hyper request was
only being used to mutate it's headers anyway.
servo/servo#6727
Because we're using unsized types not for requesting, there's not a
satisfactory way of doing this without boxing the request...
Once unsized stuff lands in rust 1.2/1.3(???) then this should be
implemented with Rc's instead of Box's.
For the time being I'm not sure what else to do.
servo/servo#6727
