When using a WWWForm in a request, it fails when parsing the headers. This is because it is looking for a KeyValuePair in a hashtable:

foreach ( KeyValuePair<string,string> pair in form.headers )
{
this.AddHeader(pair.Key,pair.Value);
}

Can be fixed by using DictionaryEntry instead:

foreach ( DictionaryEntry entry in form.headers )
{
this.AddHeader(entry.Key.ToString(), entry.Value.ToString());
}