(SERVER-39) Addressed more PR feedback

This commit includes the following changes:

1) If the `allow-header-cert-info` setting is true, only the client
   headers will be processed for client cert and DN values.  The previous
   commit would have allowed these values to be set from an SSL-provided
   certificate if one were available on the request and not superceded by
   the client headers.

2) If more than one certificate is present in the `X-Client-Cert` value
   and `allow-header-cert-info` is true, a 400 Bad Request error will be
   thrown.  The previous commit would have allowed the first certificate
   from the chain to automatically be selected and used as the end client
   certificate for the request.