Add RodaRequest#default_redirect_status, so plugins can override the default status used for redirects
Reduce object creation in head plugin
Don't set the default headers until the response is finished

Previously, since the default headers were set when the response
was initialized, you couldn't check when routing to see if the
default headers had been specifically overridden during the
response.  Setting them before finishing allows you to do that.
Add sinatra_helpers plugin, porting Sinatra::Helpers methods not covered by other plugins