Return timestamps in RFC3339 format when request includes -H "Accept: application/csv",

__Proposal:__
Return the timestamp in RFC339 format when the request includes -H "Accept: application/csv".

__Current behavior:__
As per the API documentation 1.7:

curl -H "Accept: application/csv" -G 'http://localhost:8086/query [...]

Note that when the request includes -H "Accept: application/csv", the system returns timestamps in epoch format, not RFC3339 format.

__Desired behavior:__
Return the timestamp in RFC3339 format.

__Use case:__
RFC3339 is human readable as opposed to epoch timestamps.

The request targets the /query endpoint and application/csv responses described in API documentation 1.7. Start by tracing how that endpoint formats CSV timestamps. Done means requests with Accept: application/csv return timestamps in RFC3339 format, with the behavior confirmed by tests or documentation.

Written by the indexing model from the issue text.