Nonce count should start at 1.

[martinos]

According to rfc2617 https://tools.ietf.org/html/rfc2617

    The nc-value is the hexadecimal count of the number of requests (including the current request) that the client has sent with the nonce value in this request.  For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". 

The @nonce_count instance variable is initialized to -1, it should be set to 0.

[arohr]

Yep, just stumbled over the same issue.