def get_header_tokens(headers, key):
    """
        Retrieve all tokens for a header key. A number of different headers
        follow a pattern where each header line can containe comma-separated
        tokens, and headers can be set multiple times.
    """
    toks = []
    for i in headers[key]:
        for j in i.split(","):
            toks.append(j.strip())
    return toks