Use request factory for registry ping
Use request factory for registry ping

Currently when the registry ping is sent, it creates the request directly from http.NewRequest instead of from the http request factory. The request factory adds useful header information such as user agent which is needed by the registry.

Signed-off-by: Derek McGowan <e1c79a582b6629e6b39e9679f4bb964d25db4aa8@mcgstyle.net> (github: dmcgowan)

thaJeztah/distribution,thaJeztah/distribution