src/http.php: Remove Response->contentType attribute.
src/http.php: Add constructor for Response, accepting only the response 'content' as an argument.
src/webapp/http-response.php: Make this "webapp-specific" (as opposed to web-client-specific) HttpResponse object derive from our new, unified SpareParts\HTTP\Response class.
src/test/webapp.php: Provide a more useful error message with HttpNotFound exception.
src/webapp/: Rework the way that RequestContext->takeNextPathComponent works, having it raise a 'NoMorePathComponents', rather than returning null, in the case that there are no remaining path components to consume; NoMorePathComponents then derives from PageNotFound, so as to make the default behavior for an action not finding a component it wanted to yield a 404; and, add method RequestContext->takeNextPathComponentOrNull for use in the case when a path component should actually be optional.