Changes to Ping

removed the media type text/plain for the response with the status 200

api tag System added

added the non-success response with the status 500

the optional response header Api-Version removed for the status 200

the optional response header Builder-Version removed for the status 200

the optional response header Cache-Control removed for the status 200

the optional response header Docker-Experimental removed for the status 200

the optional response header Pragma removed for the status 200

the optional response header Swarm removed for the status 200

api tag System removed

removed the non-success response with the status 500

added the media type text/plain for the response with the status 200

added the media type text/plain for the response with the status 500