Adds support for chunked responses in Curb

Calls the 'on_body' callback for each 'chunk' of the body of a webmock response if the response responds to :each and the Transfer-Encoding is 'chunked.'

This is useful for verifying that streaming requests are processing each chunk in the body of a request.

```
Finished in 37.38 seconds
3577 examples, 0 failures

Finished in 0.2438 seconds
30 examples, 0 failures

Finished in 0.021785s, 459.0314 runs/s, 596.7409 assertions/s.
10 runs, 13 assertions, 0 failures, 0 errors, 0 skips

Finished in 0.014473s, 1174.6010 runs/s, 2694.6728 assertions/s.
17 runs, 39 assertions, 0 failures, 0 errors, 0 skips
```