Support "application/json" in Blubberoid
JSON seems a better option for a web service in general—other toolchains
in the Docker/Kubernetes space typically prefer YAML for human-edited
configs but convert to JSON on the wire. The "application/json" media
type is more well established—unlike "application/yaml" which has no
official assignment by IANA—and is better supported by the OpenAPI
(formerly Swagger) specification.
Added content-type media type validation in the Blubberoid HTTP server
handler, and added a check for json.Valid(body) upon receiving a
"application/json" media type. Since any given valid JSON is also valid
YAML, Blubberoid simply does a shallow validation of the JSON body
before punting to config.ReadConfig for YAML unmarshalling and
thorough config validation.