look for -1 instead of false returned from string.indexOf

lib/multipart.js

@@ -183,7 +183,7 @@ Part.prototype.write = function(chunk) {
       var header = this.buffer.substr(0, offset).split(/: ?/);
       this.headers[header[0].toLowerCase()] = header[1];
       this.buffer = this.buffer.substr(offset+2);
-    } else if (offset === false) {
+    } else if (offset === -1) {
       return;
     }
   }