Reduce multipart memory footprint

Multipart parts kept the first chunk of data after their headers in
memory, even so that was no longer needed.

lib/multipart.js

@@ -159,6 +159,7 @@ Part.prototype.write = function(chunk) {
       this.buffer = this.buffer.substr(2);
       this.bytesReceived = this.bytesReceived + this.buffer.length;
       this.emit('body', this.buffer);
+      this.buffer = '';
       return;
     } else if (offset > 0) {
       var header = this.buffer.substr(0, offset).split(/: ?/);

test/mjsunit/test-multipart.js

@@ -28,6 +28,7 @@ var server = http.createServer(function(req, res) {
       parts[name] += chunk;
     });
     part.addListener('complete', function(chunk) {
+      assertEquals(0, part.buffer.length);
       if (parts_reveived == 1) {
         assertEquals('yes', parts[name]);
       } else if (parts_reveived == 2) {