TS-1664 check vary and accept-encoding before adding them
TS-1664 check vary and accept-encoding before adding them

This patch makes the gzip plugin check for existing vary and
accept-encoding headers before adding them. It also adds
error checking, making the compressor bail if it fail to add
the required headers

duke8253/trafficserver,dyrock/trafficserver,PSUdaemon/trafficserver,reveller/trafficserver,bryancall/trafficserver,pbchou/trafficserver,pbchou/trafficserver,duke8253/trafficserver,taoyunxing/trafficserver,vmamidi/trafficserver,chenglongwei/trafficserver,clearswift/trafficserver,reveller/trafficserver,taoyunxing/trafficserver,chitianhao/trafficserver,chitianhao/trafficserver,taoyunxing/trafficserver,chenglongwei/trafficserver,rpufky/trafficserver,vmamidi/trafficserver,taoyunxing/trafficserver,rahmalik/trafficserver,duke8253/trafficserver,vmamidi/trafficserver,PSUdaemon/trafficserver,rpufky/trafficserver,chenglongwei/trafficserver,clearswift/trafficserver,reveller/trafficserver,davidbz/trafficserver,pbchou/trafficserver,dyrock/trafficserver,persiaAziz/trafficserver,dyrock/trafficserver,persiaAziz/trafficserver,chenglongwei/trafficserver,rahmalik/trafficserver,rahmalik/trafficserver,chenglongwei/trafficserver,vmamidi/trafficserver,taoyunxing/trafficserver,persiaAziz/trafficserver,PSUdaemon/trafficserver,clearswift/trafficserver,pbchou/trafficserver,rahmalik/trafficserver,clearswift/trafficserver,taoyunxing/trafficserver,SolidWallOfCode/trafficserver,dyrock/trafficserver,SolidWallOfCode/trafficserver,rpufky/trafficserver,SolidWallOfCode/trafficserver,chenglongwei/trafficserver,SolidWallOfCode/trafficserver,persiaAziz/trafficserver,chenglongwei/trafficserver,taoyunxing/trafficserver,PSUdaemon/trafficserver,clearswift/trafficserver,rpufky/trafficserver,persiaAziz/trafficserver,PSUdaemon/trafficserver,davidbz/trafficserver,reveller/trafficserver,bryancall/trafficserver,PSUdaemon/trafficserver,PSUdaemon/trafficserver,rpufky/trafficserver,reveller/trafficserver,clearswift/trafficserver,chitianhao/trafficserver,rahmalik/trafficserver,clearswift/trafficserver,reveller/trafficserver,taoyunxing/trafficserver,rpufky/trafficserver,davidbz/trafficserver,persiaAziz/trafficserver,davidbz/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,rahmalik/trafficserver,bryancall/trafficserver,chitianhao/trafficserver,rpufky/trafficserver,dyrock/trafficserver,chitianhao/trafficserver,dyrock/trafficserver,duke8253/trafficserver,vmamidi/trafficserver,davidbz/trafficserver,persiaAziz/trafficserver,rpufky/trafficserver,dyrock/trafficserver,duke8253/trafficserver,rahmalik/trafficserver,PSUdaemon/trafficserver,reveller/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,taoyunxing/trafficserver,rahmalik/trafficserver,bryancall/trafficserver,clearswift/trafficserver,bryancall/trafficserver,clearswift/trafficserver,pbchou/trafficserver,reveller/trafficserver,chitianhao/trafficserver,reveller/trafficserver,SolidWallOfCode/trafficserver,chitianhao/trafficserver,davidbz/trafficserver,SolidWallOfCode/trafficserver,persiaAziz/trafficserver,duke8253/trafficserver,rahmalik/trafficserver,duke8253/trafficserver,PSUdaemon/trafficserver,chenglongwei/trafficserver,rpufky/trafficserver