Use instance method, not static method, and tighter method visibility.

git-svn-id: https://plugins.svn.wordpress.org/wp-seedbank/trunk@855141 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fixes #10: Expiry dates are required fields for the client.

git-svn-id: https://plugins.svn.wordpress.org/wp-seedbank/trunk@855143 b8457f37-d9ea-0310-8a92-e5e31aec5664