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 host 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.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/rack/urlmap.rb b/lib/rack/urlmap.rb
index d75f5a22..c62baecc 100644
--- a/lib/rack/urlmap.rb
+++ b/lib/rack/urlmap.rb
@@ -41,7 +41,7 @@ module Rack
     end
 
     def call(env)
-      path = env['PATH_INFO']
+      path = env[PATH_INFO]
       script_name = env['SCRIPT_NAME']
       hHost = env['HTTP_HOST']
       sName = env['SERVER_NAME']
@@ -66,7 +66,7 @@ module Rack
         return app.call(env)
       end
 
-      [404, {"Content-Type" => "text/plain", "X-Cascade" => "pass"}, ["Not Found: #{path}"]]
+      [404, {CONTENT_TYPE => "text/plain", "X-Cascade" => "pass"}, ["Not Found: #{path}"]]
 
     ensure
       env['PATH_INFO'] = path