Fix #6231 - Accept chunked encoding on start

Fixed using the @sfitts solution

I'm not exactly sure what you are wanting to check here

why not just 
```go
func isChunkedEncoding(r *http.Request) bool {
    return r.Body != nil && r.ContentLength <= 0
}
```

Do you other logic in another if or statement to make what you are checking clean and without doubt