Gets the value of header
name in this.
Use this for headers whose values are comma-delimited lists, and which are therefore allowed to appear multiple times in the headers. For non-list-valued headers, use get_one.
name appears multiple times in this, get_list will concatenate
all of the values together, separated by commas. This is sometimes awkward to parse (eg, WWW-Authenticate, Set-Cookie), but you have to be able
to deal with it anyway, because the HTTP spec explicitly states that this transformation is allowed, and so an upstream proxy could do the same
the header's value or