Allow streaming to be disabled

I'm using this middleware in Rails to set up a proxy that allows more simplified testing of an AJAX request by sending requests to the local server. Doing so allows the request to go through ruby code & to be matched via WebMock.

Basically, something like this:

 - JS library normally makes request to `api.service.com`
 - Redirect library to make requests to `localhost:3000/mock/api`
 - Rack proxy notices request for `localhost:3000/mock/api/*`, and redirects the request to host `mock.api.local`
 - WebMock is able to match requests against `mock.api.local`

This wasn't working with the streaming http requests, so I added a way of disabling them. This change should be backwards compatible for everyone who properly called super if they overrode `initialize` in their own constructor. Otherwise, it will fail to use the previous streaming logic.