Change environment variable in config for SAML2_SP_CERT_x509 to all uppercase

[f3cp]

As the title suggests, I think it might be a good idea to change the expected env var for attaching the sp certifitcate to be SAML2_SP_CERT_X509 (The X uppercase), since the convention tends to be that these variables will be all uppercase, however are treated case sensitively.

The reason I came across this is because I am using this package with Laravel Vapor, and the interface for adding 'secrets' (which are what it uses for environment variables) only allows inputing key names in uppercase which meant I need to modify the variable in the config to account for this.

While obviously most people wouldn't come across this limitation, it also seems like using all uppercase for the default config would be a reasonable convention to follow, and would avoid this and similar issues for new users of this package.

[dmyers]

This is now complete with PR #36 being merged.