Allow sending custom headers on `fetch` method.

[aaronfc]

Currently the fetch method is not allowing almost any customization.

I found some webpages in which my requests were blocked because we were not sending the User-Agent header.

To do so, I copy pasted the fetch method and added the curl_setopt($curl, CURLOPT_USERAGENT, 'My User Agent/1.0'); option.

Alternatively, I think that sending an extra header like 'User-Agent: My User Agent/1.0' in the existing CURLOPT_HTTPHEADER option should have worked too.

I could have also done the request by myself and then used the parse method, but fetch includes some extra logic (checking content_type` and making sure the 'url' used is the correct one, that I would have had to replicate.

Proposal:

• Make it possible to add extra headers (maybe as an optional extra parameter which is [] by default).