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!
@daniel-g there are at least 3 incorrect scenarios we are testing for, hence the 3 versions of the invalid certificate. As with my other PR, my goal was not to rewrite or change large portions of the codebase, it was to solve a problem within the parameters provided, and this PR follows the existing code style and patterns. 

Again, I'd be happy to do some further refactoring if that's what OneLogin folks are looking for, but this library needs a lot of that and I would think that should be done outside of this PR.