There should be for_each_value_of method in http_header_fields_t class

[eao197]

For now http_header_fields_t class contains only methods for getting the value of a single occurrence of a field (like get_field, try_get_field, get_field_or, value_of, opt_value_of). But HTTP-fields can be repeated in a HTTP message. To handle such repeated fields it's necessary to have something like that:

headers.for_each_value_of(
   restinio::http_field_t::accept,
   [](restinio::string_view_t value) {...});
...
headers.for_each_value_of(
   "Accept",
   [](restinio::string_view_t value) {...});