Allow algorithm in certbot_dns_rfc2136's config to be case insensitive

Update dns_rfc2136_test to use a mixed-case test value in the valid algorithm test.

certbot-dns-rfc2136/certbot_dns_rfc2136/dns_rfc2136.py

@@ -57,7 +57,7 @@ class Authenticator(dns_common.DNSAuthenticator):
     def _validate_algorithm(self, credentials):
         algorithm = credentials.conf('algorithm')
         if algorithm:
-            if not self.ALGORITHMS.get(algorithm):
+            if not self.ALGORITHMS.get(algorithm.upper()):
                 raise errors.PluginError("Unknown algorithm: {0}.".format(algorithm))
 
     def _setup_credentials(self):

certbot-dns-rfc2136/certbot_dns_rfc2136/dns_rfc2136_test.py

@@ -64,7 +64,7 @@ class AuthenticatorTest(test_util.TempDirTestCase, dns_test_common.BaseAuthentic
 
     def test_valid_algorithm_passes(self):
         config = VALID_CONFIG.copy()
-        config["rfc2136_algorithm"] = "HMAC-SHA512"
+        config["rfc2136_algorithm"] = "HMAC-sha512"
         dns_test_common.write(config, self.config.rfc2136_credentials)
 
         self.auth.perform([self.achall])