Fix returned EOF error when calling Nodes GC/GcAlloc API

[jrasell]

Internally Nomad returns a http.NoBody when calling nodes GC, therefore attempting to decode the body results in an EOF error returned to the user. This fix updates the decodeBody function to check both the response content length and the passed out object.

The following code snippet can be used to locally test:

package main

import (
	"fmt"

	"github.com/hashicorp/nomad/api"
)

func main() {
	nomadClient, _ := api.NewClient(api.DefaultConfig())
	nodeSelf, _ := nomadClient.Agent().Self()
	fmt.Println(nomadClient.Nodes().GC(nodeSelf.Stats["client"]["node_id"], nil))
}

Closes #5506 #5994

[tgross]

@langmartin it looks like the requested changes were made, but I can't get Travis to rebuild it because we've removed the scripts for Travis since this was last pushed. Maybe if @jrasell would be willing to squash his commits and re-push, it'll trigger CircleCI and we can get this merged?

[langmartin]

Yes! @jrasell the age on this is my fault, it got mixed up with some followup work: we shouldn't be using side effecting GET requests for making changes. I'll capture that in another ticket, and this should be fine to merge. @tgross suggestion to squash is right. Sorry about letting this fall through the cracks!

[notnoop]

Should we always skip decoding body if out == nil? I noticed that we wrap few decodeBody invocations with a nil check already:

nomad/api/api.go

Lines 858 to 862 in d890ddb

	if out != nil {
	if err := decodeBody(resp, &out); err != nil {
	return nil, err
	}
	}

.

It would simplify caller code a bit, and it doesn't seem like an error if caller explicitly passes nil and ignores response body (though body may need to be consumed... a potential bug in current code).

[github-actions]

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