optimized getHeaders/getCookies for perf

By profiling on Mac/Linux, I've seen that we had some hotspots in core/server/Helpers.scala and api/mvc/Http.scala due to getHeaders/getCookies which are called for each requests. Not so big hotspots but I saw the code could be optimized.
I tried different things with more or less perf gains but more or less intrusive also (trading big gain vs memory loss wasn't so good).
Here is a modif which provides a gain of >500req/s (around 600-700) on my MacPro Retina/JDK7 without breaking anything.

This is not so much but anyway let's take it ;)