header_filter_by_lua_block can not unset header if set by more_set_headers

[mvorisek]

header_filter_by_lua_block can not unset header if set by more_set_headers

Example:

more_set_headers 'test: test';

header_filter_by_lua_block {
    -- some custom logic to unset previously set header from
    -- upstream or by more_set_headers
    if 1 = 1 then
        ngx.header["test"] = nil
    end
}