HTTP Digest authentication for ActiveResource

ActiveResource supports HTTP Basic authentication, but not HTTP Digest authentication, which poses something of a problem when consuming APIs that expect the client to provide Digest credentials (as currently supported by Rails). This patch allows an ActiveResource class to specify that it should provide credentials to the server as a digest. As an example:

class Foo < ActiveResource
self.user = 'fred' self.password = 'password' self.auth_type = :digest end

Any auth_type other than 'digest' or :digest will default back to Basic authentication. If the class doesn't specify credentials it still falls back to no authentication. Tests included.