detect seekable wsgi.input and use it instead of making another copy

[mmerickel]

Waitress explicitly provides a seekable input object, and thus webob may not need to directly copy it. I'm not sure how safe / reliable it is to feature detect this situation, but if it provides seek/tell methods then it's probably worth relying on.

related: Pylons/pyramid#3599

[ztane]

I guess one should try to seek it too. I could imagine it being some kind of pipe or similar that does have seek because it just does, but seek throwing error.