handle 'continue' event only once

Linode Object Storage sends multiple '100 continue' lines as a response
to 'Expect: 100-continue'. This causes node.js to parse the response
correctly and fire the 'continue' event twice.

See also aws#3404

lib/http/node.js

@@ -98,7 +98,7 @@ AWS.NodeHttpClient = AWS.util.inherit({
 
     var expect = httpRequest.headers.Expect || httpRequest.headers.expect;
     if (expect === '100-continue') {
-      stream.on('continue', function() {
+      stream.once('continue', function() {
         self.writeBody(stream, httpRequest);
       });
     } else {