Bug #1305: Double content-length often harmless

Bug #1305: Double content-length often harmless

There is quite many web servers out there with broken banner engines
forgetting to delete the original content-length after adding the
banner. Currently these are (rightfully) rejected by Squid.

Instead of rejecting we could select the biggest content-length header
found and remove the other. This should fix up these replies while not
allowing for attacks.

Forward port of 2.5 patch.