Less allocated objects on each request

How many? Using `memory_profiler` and a Rails app (codetriage.com), master uses:

```
rack/lib x 7318
```

After this patch, the app uses:

```
rack/lib x 4598
```

Or `(7318 - 4598) / 7318.0 * 100 # => 37.16` % fewer objects __PER REQUEST__.

To do this, I extracted really commonly used strings into top level Rack constants. It makes for a bit of a big diff, but I believe the changes are worth it. 

Running benchmark/ips against the same app, I'm seeing a performance boost of `2~4%` across the entire app response. This doesn't just make Rack faster, it will make your app faster.

While we could certainly go overboard and pre-define ALL strings as constants, that would be pretty gnarly to work with. This patch goes after the largest of the low hanging fruit.
Fix is here: https://github.com/rack/rack/pull/739 sorry about that, should have known better. Thanks for the catch!