• Increasing default MDRetryDelay to 30 seconds to generate less bursty
  traffic on errored renewals for the ACME CA. This leads to error retries
  of 30s, 1 minute, 2, 4, etc. up to daily attempts.
• Checking that configuring MDRetryDelay will result in a positive
  duration. A delay of 0 is not accepted.
• Fix a bug in checking Content-Type of responses from the ACME server.