MINOR: http/harderning: implement "delete-underscore-in-header-name"

MINOR: http/harderning: implement "delete-underscore-in-header-name"

Deletes from requests and responses any header field whose name
contains an underscore character ('_' or \x5C). While this character is
perfectly permitted, some application frameworks which create automatic
variables already turn the dash ('-') into an underscore in variable
names, leading to an ambiguity with the underscore which can result in
the same variable name. This could be abused to bypass some
access-control policy for example. No standard HTTP header uses this,
so it is recommended to enable on untrusted listeners. Note that this
applies only after "reject-underscore-in-header-name" was accepted.