Make #auth_capable? public

This can be useful to users of the library, in the same way that
`#capable?`, `#capabilities`, and `#capable_auth_types` are all useful.
I suggest deprecating the specific mechanism predicate methods
(`capable_plain_auth?`, `capable_login_auth?`, and
`capable_cram_md5_auth?`), as they are all replaced by this.

lib/net/smtp.rb

@@ -316,12 +316,13 @@ def capable_cram_md5_auth?
       auth_capable?('CRAM-MD5')
     end
 
+    # Returns whether the server advertises support for the authentication type.
+    # You cannot get valid result before opening SMTP session.
     def auth_capable?(type)
       return nil unless @capabilities
       return false unless @capabilities['AUTH']
       @capabilities['AUTH'].include?(type)
     end
-    private :auth_capable?
 
     # Returns supported authentication methods on this server.
     # You cannot get valid value before opening SMTP session.