use closure instead of .bind()

i've run the benchmark several times in my laptop. there is a slightly performance improve.

node v0.11.12.(test on 0.11.13, bind is slower than closure as well)
before

```
  1 middleware
  9361.78

  5 middleware
  9277.84

  10 middleware
  9166.20

  15 middleware
  8954.21

  20 middleware
  8771.13

  30 middleware
  8590.82

  50 middleware
  8118.87

  100 middleware
  7134.31
```

after

```
  1 middleware
  9690.68

  5 middleware
  9637.66

  10 middleware
  9449.17

  15 middleware
  9285.14

  20 middleware
  8922.27

  30 middleware
  8755.52

  50 middleware
  8418.54

  100 middleware
  7242.11
```