Do not parse json on 204 responses (#476)

* Do not return BadResponse for no content 

Signed-off-by: Adam Jaso <2285656+adamjaso@users.noreply.github.com>

api/prometheus/v1/api.go

@@ -477,12 +477,14 @@ func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, [
 
 	var result apiResponse
 
+	if http.StatusNoContent != code {
 		if err = json.Unmarshal(body, &result); err != nil {
 			return resp, body, &Error{
 				Type: ErrBadResponse,
 				Msg:  err.Error(),
 			}
 		}
+	}
 
 	if apiError(code) != (result.Status == "error") {
 		err = &Error{