Ensure IdP certificate is formatted properly

When loading the provider certificate from the environment, or integrating via `omniauth-saml`, we would often encouter a series of _nested asn1 error_ messages due to the malformed certificate failing to be read by `OpenSSL`.

Inspiration for this patch comes from toyokazu/rack-saml@b7646ad

Thanks to @dannyb for the assistance!
These 2 methods should belong to another class that handles certificates