Remove the tree map in the HTTP headers.

This map is intended make looking up headers faster, but for real-world
sized datasets we spend more energy maintaining the map than we save from
using it.

I benchmarked the Android Market's HTTP connections' header use and it's
significantly faster to skip the map:

      run    us linear runtime
 map+list 413.1 ==============================
list only  65.8 ====

Change-Id: I5b8fef6f999eb1bfe53eea2781f63cdd1002a273
5 files changed