(SERVER-39) Allow a certificate to be passed into a request via X header

This commit adds the ability for a certificate to be supplied in a web
request as the payload of a custom HTTP header, X-Client-Cert.  The
header will only be parsed for a value if the `allow-header-cert-info`
master setting is set to true.  When the certificate is retrieved, it
will be supplied as the client certificate to the Ruby request handler
stack, as a direct replacement for a certificate object that would be
typically be provided over SSL.
Thanks, I didn't know about `thrown+?`.  I'll rework it to use that.  FYI - I had just lifted the previous code from here and wasn't happy about having done that anyway:

https://github.com/puppetlabs/puppet-server/blob/master/test/puppetlabs/puppetserver/certificate_authority_test.clj#L80