Methods that take both PEM-encoding and DER-encoding have not been
consistent in the order in which encoding to attempt to parse.

A DER-encoding may contain a valid PEM block ("\n-----BEGIN ..-----" to
"-----END ...-----") embedded within it. Also, the PEM-encoding parser
allows arbitrary data around the PEM block and silently skips it. As a
result, attempting to parse data in DER-encoding as PEM-encoding first
can incorrectly finds the embedded PEM block instead.

This commit ensures that DER encoding will always be attempted before
PEM encoding. OpenSSL::X509::Certificate is one of the updated classes.
With this, the following will always be true:

    # obj is an OpenSSL::X509::Certificate
    obj == OpenSSL::X509::Certificate.new(obj.to_der)
    obj == OpenSSL::X509::Certificate.new(obj.to_pem)

https://github.com/ruby/openssl/commit/b280eb1fd0