Fix problem with multiple requests like:

var twitter=new TinyTwitter.TinyTwitter(oauth);
twitter.UpdateStatus(".");
twitter.UpdateStatus("..");
twitter.UpdateStatus("..");
twitter.UpdateStatus("....");
twitter.UpdateStatus(".....");

In this case only ».« and »..« are pushed to 
Twitter. After »..« the connections hangs. The
Problem is that HttpWebResponse must be close
after reuse.

HttpWebResponse is now close after use.