IWebServiceConfiguration.set_hop_by_hop_headers broke backward compatibility

IWebServiceConfiguration recently grew an attribute set_hop_by_hop_headers, but this was done in a backwards incompatible way. My lazr.restful application (mailman) just grabbed the latest lazr.restful and broke because my IWebServiceConfiguration did not have this attribute.

The easy fix in this case is to use getattr() in applyTransferEncoding, defaulting the value to False if the attribute is missing.

You might argue that at 0.9.13, the API is not guaranteed to be stable.

You might argue that I really should be using BaseWSGIWebServiceConfiguration.

You might be right, and yet it would still be good to advertise the change better. There's no mention in the NEWS file about requiring this additional attribute.