pem: do not exclude self-signed certificates when encoding X509 chain

[lrascao]

Description

RFC 5246 states that:

      Because certificate validation requires that root keys be distributed
      independently, the self-signed certificate that specifies the root
      certificate authority MAY be omitted from the chain, under the
      assumption that the remote end must already possess it in order to
      validate it in any case.

Since it's not saying they MUST be omitted we can keep the self-signed certs and keep symmetry in the encode/decode functions.