Reopened Issue: Problem with Encoding ParseContentType(string value, out string contentType) ParseHelper.cs [1548]

Ocorred error in line
```
return Encoding.GetEncoding(tmp.Groups[tmp.Groups.Count - 1].Value.Split(new[] { ';' })[0].Trim());
```

__'utf8' is not a supported encoding name. Parameter name: name__

Should be 'utf-8' and not 'utf8'.

I create simple workaround for this.

```
var e = Encoding.UTF8;

try
{
e = Encoding.GetEncoding(tmp.Groups[tmp.Groups.Count - 1].Value.Split(new[] { ';' })[0].Trim());
}
catch (Exception) { }

return e;
```

Someone had the same problem?

Thks,
Helio Elias