(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.
I have found in the past that putting assertions inside a helper function is kind of a drag; it means that a failure will show up with a line number that doesn't immediately make it clear which test caused the failure.  It also means that if you're using Cursive to run your tests that you can't rely on the visual indicators to help you see which test failed.  Could we maybe just make this into like a predicate function, and then include `(is (true? (blah...)))` assertions in the actual tests?