enhanced redirect logging

This adds conditional logic for logging the `'Location'` header
if it is present.  This allows you to see the location for any
redirects that happen in the logs.

@jcredding any problems with this.  The "controversial" things could be: conditionals in the logging middleware (performance impact?) and only showing keys that exist in the summary logging (although as I type this that shouldn't be controversial b/c we always populate keys for the non-redir stuff even if they have no value).  Anyway, ready for review.