clogger 0.4.0

Small cleanups and minor enhancements (mainly for the optional
C extension) for this release:

Eric Wong (10):
      ext: use FIX macros instead of generic NUM macros if possible
      ext: preserve subclasses of Rack::Utils::HeaderHash
      test: fix Linux memory usage test
      avoid direct require of "rack" to quiet warnings
      ext: GC safety fix when throwing an exception
      ext: eliminate unused variable
      ext: fix signedness and shadow warnings
      ext: sparse cleanups
      ext: avoid void pointer arithmetic
      clogger 0.4.0