Set content-type to application/yang-data+json as specified in RFC8040

[sebastien-guay]

The current content-type in a POST response is set to application/json.

According to RFC8040 it must be application/yang-data+xml or application/yang-data+json. In our case it is application/yang-data+json.

See https://www.rfc-editor.org/rfc/rfc8040.html#section-3.2

[dhubler]

merged (with minor tweak) thanks!
Ideally squash commits before submitting. If I squash them, then it erases author tag.

[sebastien-guay]

Thanks!