(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 covered this a bit in the response to @jmay's comment below.  I don't have a good use case for why/when you'd want to mix and match these.  I could have left the custom headers out of this test case and probably achieved what I was looking for here.

I could see a case where someone might configure the service to be hosted by the webserver under multiple ports - one plaintext, one SSL.  For the plaintext one, you might need to configure the "allow-header-cert-info" to "true" in order to pass through the X- headers.  I was mostly looking to validate that if you provided a client cert through SSL that it would still be honored -- assuming you didn't "override" it with an X-Client-Cert value.

Wasn't sure I wanted to get into varying how the "allow-header-cert-info" setting is honored based on whether the client connected via plaintext or SSL.  That would seem tricky.

If we were to get into validating that the different DNs all agree with one another and were to see a difference, I'd be more inclined to throw an error - to make the problem more obvious - than to just passively substitute `nil` into the JRuby request.  If we were going to do that much, probably would make sense to compare the "certs" -- HTTP header one vs. SSL-provided one -- to see if they are the same as well.

Dunno...  Do we want to hold this PR up to work out the credential comparison stuff?  Seems like there were already potential holes with this in the Ruby implementation since the X-Client-DN did not necessarily have to agree with the DN in whatever happened to be provided on the SSL_CLIENT_CERT environment variable via Apache.