The HTTP "Accept" header is interpreted in a non-standard way

InfluxDB does not seem to interpret the [Accept HTTP header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept) according to the HTTP specification. It seems to return a MessagePack encoded response only when the header has the exact value `application/x-msgpack`. Adding a second accepted format to the header makes influxdb switch back to returning JSON.

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Install influx
2. From a terminal, run
```sh
curl -G 'http://localhost:8086/query' --data-urlencode 'q=SHOW databases' --header "Accept: application/x-msgpack, text/plain"
```

__Expected behavior:__
A MessagePack response is returned.

__Actual behavior:__
A JSON response is returned.

__Environment info:__

* InfluxDB version: InfluxDB vv1.7.6 (git: master 01c8dd416270f424ab0c40f9291e269ac6921964)

Start by reproducing the curl request against InfluxDB v1.7.6 and compare the response for a single Accept value versus `application/x-msgpack, text/plain`. Trace the HTTP query endpoint's Accept-header handling; done means a header listing multiple accepted formats still selects MessagePack when it is supported, with regression coverage for the reported command.

Written by the indexing model from the issue text.