Fix skewed middleware benchmarks (#165)

[jub0bs]

Fixes #165. I have not updated the Benchmarks section of the README (you may want to do that yourself after running the benchmarks on your own machine).

[rs]

The updated benchmark is, in fact, more skewed as it assumes the header map is not initialized. It allocates the first key to this library, whereas in reality all responses will inherently come with at least one header. I'm going to push a modified version of this PR with a header initialized. Thanks.

[jub0bs]

@rs

Interesting. I'm not sure what a good middle ground would be. All I know is that CORS middleware typically get stacked at or close to the top (they're the first in the middleware chain to execute) and that the http.Header map used under the hood by http.ResponseWriter initially has zero capacity (no size hint).