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

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

This is the same as for the headers, but this time for the trailers. Better
keep them separated as trailers are used even worse than headers these days.

Note that in h1 we delete them when we detect them since we already have
the code for this, no need to index them and delete them later.