Fix #53: empty passphrase breaks signing provider

If configuration defines the passphrase for the key as either null or an empty string, the test `isset($config['key_pass'])` returns false. Since `key_pass` is optional, this commit removes it from the test and instead provides a default value if it's not set.
Sign AuthNRequest

Note: I have no idea if this explodes when signing is disabled or not configured.
Don't sign unless the signing provider is enabled
Merge pull request #56 from inanimatt/master

Fix #53: empty passphrase breaks signing provider