set default encoding in sendBody

lib/http.js

@@ -205,6 +205,7 @@ OutgoingMessage.prototype.sendHeaderLines = function (first_line, headers) {
 };
 
 OutgoingMessage.prototype.sendBody = function (chunk, encoding) {
+  encoding = encoding || "ascii";
   if (this.chunked_encoding) {
     this.send(process._byteLength(chunk, encoding).toString(16));
     this.send(CRLF);