SP Metadata ValidUntil shouldn't default to 7 days

[stevenao]

I think it is better to have an option to specific the time the generated metadata valid for, or simply not showing it in the metadata at all. I have done the code change in my fork.

https://github.com/stevenao/SAML2/commit/eba7f23ce2a76379492a82f6ce233954f357a894