Improve allocation speed and reduce memory copying to increase HTTP pars...

...er performance by 1-2 (and occasionally more) percent in many cases such as benchmark/http/end-vs-write-end.js based on slab allocator todo by Ben.
Don't use `String#indexOf()` if you just want to check if the first n bytes match a pattern.  Better (if more laborious) is to write it as something like:

```javascript
var pattern = 'HTTP/1.1 200 OK\r\n';
if ((d.length === pattern.length && d === pattern) ||
    (d.length > pattern.length && d.slice(0, pattern.length) === pattern)) {
  // ...
}
```