fix unnecessary binary->string conversion

lib/HeaderHostTransformer.js

@@ -19,11 +19,11 @@ util.inherits(HeaderHostTransformer, Transform);
 
 HeaderHostTransformer.prototype._transform = function (chunk, enc, cb) {
     var self = this;
-    chunk = chunk.toString();
 
     // after replacing the first instance of the Host header
     // we just become a regular passthrough
     if (!self.replaced) {
+        chunk = chunk.toString();
         self.push(chunk.replace(/(\r\nHost: )\S+/, function(match, $1) {
             self.replaced = true;
             return $1 + self.host;