Failing spec for missing resource response on non-existent route.
add Webmachine::ContentNegotiation for Webmachine.render_error().

closes #158
add Webmachine::HeaderNegotiation, replaces Webmachine::ContentNegotiation.
set 'Date' header on render of 404 from Webmachine.render_error()
add regression test for #155
Merge pull request #159 from seancribbs/fix-404

ensure Webmachine.render_error() sets 'Content-Length'.