There are multiple ways form data can be encoded. 'multipart' is
just one of them - there are also 'application/x-www-form-urlencoded'
(which will be added in next commit) and 'text/plain' encodings.

Let each encoding have it's own form_xxxx file and tests.

This commit adds transport that handles form POST with content
type set to 'application/x-www-form-urlencoded'.

Form body can be json, urlencoded parameters or plain text.

Example:

```
 curl -X POST 'http://server/query' -d '{name}' -H "Content-Type: application/x-www-form-urlencoded"
```

Enable it in your GQL server with:

```
srv.AddTransport(transport.UrlEncodedForm{})
```

No other changes but this rewrite to switch.