Yum Proxy implement Condition GET

When requesting from a Yum Proxy we should provided a proper response for a conditional get with the If-Modified-Since header.

Acceptance Criteria

• Add for the URI patterns in YumProxyRecipe to add a Conditional Request Handler, affected URL's should return a 304 http status if data was not modified before the date time given in the If-Modified-Since header. 
  • /repomd.xml
  • /repodata/*
  • *.rpm
  • *.xml.asc

Example call

curl -v -H "If-Modified-Since: Fri, 04 Aug 2017 16:45:35 GMT" http://localhost:8081/repository/yum-proxy/repodata/repomd.xml