Add Key#expires? and fix SubKex#expires

- Add SubKey#expires?

  Returns true if the subkey has an expiration date, else false

- Fix SubKey#expires

  Returns nil if the key has no expiration date

- Add Key#expires?

  Returns true if the key has an expiration data, else false

lib/gpgme/key.rb

@@ -151,6 +151,12 @@ def delete!(allow_secret = false)
       end
     end
 
+    ##
+    # Returns true if the key has an expiry date else false
+    def expires?
+      primary_subkey.expires?
+    end
+
     ##
     # Returns the expiry date for this key
     def expires

lib/gpgme/sub_key.rb

@@ -11,13 +11,16 @@ def timestamp
       Time.at(@timestamp)
     end
 
+    def expires?
+      @expires != 0
+    end
+
     def expires
-      Time.at(@expires)
+      expires? ? Time.at(@expires) : nil
     end
 
     def expired
-      return false if @expires == 0
-      @expires < Time.now.to_i
+      expires? && @expires < Time.now.to_i
     end
 
     def sha