Add test implementation for Digest authentication
Add test implementation for Digest authentication

It can be used for testing of all available combinations of qop (auth/auth-int/not specified) and algorithms (MD5, SHA-256).

pestanko/httpbin,ewdurbin/httpbin,postmanlabs/httpbin,kennethreitz/httpbin,kennethreitz/httpbin,pestanko/httpbin,ewdurbin/httpbin,Runscope/httpbin,krissman/httpbin,krissman/httpbin,Runscope/httpbin,postmanlabs/httpbin