GET render with content-type header results in an error

[Dieterbe]

This is easily verified using docker-dev.

A content-type header on a GET request which has no body isn't useful, but the http RFC seems to allow it. Graphite allows it too, so we should allow it too.
(without the header specified, it works fine)

graphite

http  'http://localhost:8080/render?target=stats.docker-env.response.200&from=-6s&format=json' 'Content-Type:application/json'
HTTP/1.1 200 OK
Cache-Control: no-cache, no-store, must-revalidate, max-age=0
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 137
Content-Type: application/json
Date: Wed, 18 Sep 2019 14:28:01 GMT
Expires: Wed, 18 Sep 2019 14:28:01 GMT
Keep-Alive: timeout=5, max=100
Server: Apache/2.4.18 (Ubuntu)
Vary: Accept-Encoding
X-Graphite-Hostname: graphite
X-Graphite-Timing: D=91222 t=1568816881642327

[
    {
        "datapoints": [
            [
                null,
                1568816876
            ],
            [
                1.0,
                1568816877
            ],
            [
                null,
                1568816878
            ],
            [
                null,
                1568816879
            ],
            [
                null,
                1568816880
            ],
            [
                null,
                1568816881
            ]
        ],
        "tags": {
            "name": "stats.docker-env.response.200"
        },
        "target": "stats.docker-env.response.200"
    }
]

metrictank

http  'http://localhost:6060/render?target=stats.docker-env.response.200&from=-6s' 'Content-Type:application/json'
HTTP/1.1 422 Unprocessable Entity
Content-Encoding: gzip
Content-Length: 96
Content-Type: application/json; charset=utf-8
Date: Wed, 18 Sep 2019 14:29:00 GMT
Vary: Accept-Encoding

[
    {
        "classification": "RequiredError",
        "fieldNames": [
            "target"
        ],
        "message": "Required"
    }
]