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!