bots: Handle Last-Modified in GitHub caching
bots: Handle Last-Modified in GitHub caching

Handle Last-Modified headers properly in GitHub caching as
described here:

https://developer.github.com/v3/#conditional-requests

Closes #6678
Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

deryni/cockpit,andreasn/cockpit,petervo/cockpit,andreasn/cockpit,mvollmer/cockpit,Scribery/cockpit,garrett/cockpit,xhad/cockpit,petervo/cockpit,larsu/cockpit,mareklibra/cockpit,harishanand95/cockpit,deryni/cockpit,cockpit-project/cockpit,harishanand95/cockpit,Scribery/cockpit,mareklibra/cockpit,harishanand95/cockpit,larskarlitski/cockpit,andreasn/cockpit,larsu/cockpit,cockpituous/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,harishanand95/cockpit,stefwalter/cockpit,dperpeet/cockpit,larsu/cockpit,dperpeet/cockpit,cockpituous/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,larsu/cockpit,cockpituous/cockpit,michalskrivanek/cockpit,martinpitt/cockpit,Scribery/cockpit,michalskrivanek/cockpit,michalskrivanek/cockpit,deryni/cockpit,mareklibra/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,larsu/cockpit,andreasn/cockpit,moraleslazaro/cockpit,petervo/cockpit,stefwalter/cockpit,mvollmer/cockpit,cockpituous/cockpit,larskarlitski/cockpit,xhad/cockpit,martinpitt/cockpit,martinpitt/cockpit,andreasn/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,larskarlitski/cockpit,michalskrivanek/cockpit,xhad/cockpit,cockpit-project/cockpit,petervo/cockpit,Scribery/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,martinpitt/cockpit,garrett/cockpit,Scribery/cockpit,michalskrivanek/cockpit,dperpeet/cockpit,cockpit-project/cockpit,martinpitt/cockpit,dperpeet/cockpit,garrett/cockpit,petervo/cockpit,xhad/cockpit,michalskrivanek/cockpit,dperpeet/cockpit,larsu/cockpit,SotolitoLabs/cockpit,garrett/cockpit,harishanand95/cockpit,xhad/cockpit,Scribery/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,mvollmer/cockpit,petervo/cockpit,dperpeet/cockpit,stefwalter/cockpit,andreasn/cockpit,larskarlitski/cockpit,larskarlitski/cockpit,harishanand95/cockpit,Scribery/cockpit,stefwalter/cockpit,garrett/cockpit,xhad/cockpit,SotolitoLabs/cockpit,larskarlitski/cockpit,cockpit-project/cockpit,larskarlitski/cockpit,deryni/cockpit,mareklibra/cockpit,stefwalter/cockpit,deryni/cockpit,xhad/cockpit,deryni/cockpit,dperpeet/cockpit,petervo/cockpit,harishanand95/cockpit,michalskrivanek/cockpit,stefwalter/cockpit