Encoding detection fix

[blagoySimandov]

Connected to issue #777 "HTML encoding is not autodetected properly". I removed the current gocolly encoding detection, which through tests showed to be unreliable when detecting Cyrillic encodings, and in place of it used the built-in function DetermineEncoding from the charset package.

[WGH-]

charset.DetermineEncoding returns the detected encoding, so I think this entire function can be greatly simplified and largely replaced with rougly this:

enc, _, certain := charset.DetermineEncoding(r.Body, contentType)
if !certain && !detectCharset {
	return nil
}
var err error
r.Body, err = ioutil.ReadAll(enc.NewDecoder().Reader(bytes.NewReader(r.Body)))
return err

[WGH-]

This needs a unit test.

[WGH-]

Thanks! This needs a couple of changes, though (see above).

And please clean up your commit history (or I will do it myself before the merge)