pki: Always and implicitly use base64 encoding for EST requests/response

pki: Always and implicitly use base64 encoding for EST requests/response

Content-Transfer-Encoding is actually not a valid HTTP header, but a MIME
header, and must not be used. The original RFC7030 specifies this wrong,
and an errata discusses this issue.

The use of base64 encoding has been clarified in RFC8951, and the
recommendation is to always use/expect base64 encoding, but not send/expect
the Content-Transfer-Encoding header.