This function was making a request to the `_ping` endpoint, which (if
implemented) would return a JSON response, which we unmarshal (the only
field we use from the response is the `Standalone` field).
However, if the response had a `X-Docker-Registry-Standalone`, that header
took precedence, and would overwrite the earlier `Standalone` value we
obtained from the JSON response.
This patch adds a fast-path for situations where the header is present,
in which case we can skip handling the JSON response altogether.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
e9ad878df6