GetFileMetaData errors on missing file

[Pie4kaiz]

When calling the RepositoryFilesService.GetFileMetaData() function on a file that doesn't exist, but the request was successful, I expect the returned *File and error to be nil.
Instead I get an ErrorResponse containing HEAD http://127.0.0.1:49996/api/v4/projects/1234/repository/files/foobar.yaml: 404 failed to parse unknown error format.

This is due to the fact that CheckResponse() (called by Client.Do(), called by RepositoryFilesService.GetFileMetaData()) tries to parse the empty body as JSON.
I think it fails here:

data, err := ioutil.ReadAll(r.Body)
	if err == nil && data != nil {

data doesn't seem to be nil here, maybe it contains \r\n? As I can see, this endpoint that came with #511 is the only one within the codebase using HEAD, which is probably why it never popped up before.
Nevertheless I think that a HEAD request that returns 404 should either be not an error or some specific error (e.g. NotFoundError) to be catchable.