fix postfield logic,make it easy to use

Before,when you want to use 'application/x-www-form-urlencoded',you must write

    $a = array('a'=>'1','b'=2);
    $curl->post('http?/google.com',http_build_query($a));

Now,when you write
    
    $curl->post('http://google.com',array('a'=>'1','b'=2));

It use 'application/x-www-form-urlencoded'

when you write

    $curl->post('http://google.com',array('a'=>'1','b'='@/test.jpg'));

It use 'multipart/form-data'