Allow case-insensitive strings for SASL mechanism

In particular, this erases the difference between :scram_sha_256 (the
Net::SMTP authtype) and "SCRAM-SHA-256" (the SASL mechanism).

Author: nevans

lib/net/smtp/authenticator.rb

@@ -6,11 +6,13 @@ def self.auth_classes
       end
 
       def self.auth_type(type)
+        type = type.to_s.upcase.tr(?_, ?-).to_sym
         Authenticator.auth_classes[type] = self
       end
 
       def self.auth_class(type)
-        Authenticator.auth_classes[type.intern]
+        type = type.to_s.upcase.tr(?_, ?-).to_sym
+        Authenticator.auth_classes[type]
       end
 
       attr_reader :smtp

test/net/smtp/test_smtp.rb

@@ -485,7 +485,7 @@ def test_start_auth_cram_md5
       omit "openssl or digest library not loaded" unless defined? OpenSSL or defined? Digest
 
       port = fake_server_start(auth: 'CRAM-MD5')
-      Net::SMTP.start('localhost', port, user: 'account', password: 'password', authtype: :cram_md5){}
+      Net::SMTP.start('localhost', port, user: 'account', password: 'password', authtype: "CRAM-MD5"){}
 
       port = fake_server_start(auth: 'CRAM-MD5')
       assert_raise Net::SMTPAuthenticationError do