do not sanitize HTTP/2 pseudo-headers

Per https://github.com/elastic/apm/pull/575 the `sanitize_field_names` patterns should no longer apply to captured HTTP/2 *pseudo-headers*. Practically this means that we should no longer expect the `:authority` HTTP/2 pseudo-header to be redacted by the `*auth*` wildcard pattern that is part of the default `sanitize_field_names` value.