Use `unpack1` instead of `unpack` and `[0]`

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59381 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/smtp.rb

@@ -1035,9 +1035,9 @@ module Net
       end
 
       # Creates a CRAM-MD5 challenge. You can view more information on CRAM-MD5
-      # on Wikipedia: http://en.wikipedia.org/wiki/CRAM-MD5
+      # on Wikipedia: https://en.wikipedia.org/wiki/CRAM-MD5
       def cram_md5_challenge
-        @string.split(/ /)[1].unpack('m')[0]
+        @string.split(/ /)[1].unpack1('m')
       end
 
       # Returns a hash of the human readable reply text in the response if it