Incorrect schema for network-traffic http-request-ext request_header

[JasperJuergensen]

The standard for STIX2.1 specifies for the http-request-ext for the request_header field:

The corresponding value for each dictionary key MUST always be a list of type string to support when a header field is repeated.

The schema enforces string values instead of lists of strings. https://github.com/oasis-open/cti-stix2-json-schemas/blob/5353a26a5078d7967a1bb4abb8406a75bffd5dff/schemas/observables/network-traffic.json#L201

This causes problems when using the stix-validator:

network-traffic--a6cb5584-ba80-5838-a35a-daed632b1ee6: extensions.http-request-ext.request_header.Host: ['example.com'] is not of type 'string'