add/update javadoc based on investigation of ARI-3777.
cleanup code around ARI-3777
  remove unused imports/code lines
  replace deprecated import with new
add test capturing ARI-3777 - Transfer-Encoding: chunked header gets passed into response in proxy mode.
remove Transfer-Encoding header even in IdentityHttpHeaderProcessor. Fixes ARI-3777.
  consolidated common functionality of preserving original headers by prefixing into new base class - this changes behavior for corner cases of prefix being null or "" (now more consistent).