FIXES #5: accepts quoted value for header field 'algorithm'

I stumbled upon an issue with the algorithm header field. It seems to me that ruby is quoting the header values and therefor produces an invalid value resulting in the Exception described in https://github.com/drbrain/net-http-digest_auth/issues/5.
Just to be sure I checked the server response and everything seems to be fine here (i.e algorithm=MD5).
This commit enables the gem to accept both, quoted and unquoted values.