Provides access to an unstructured header field

Per RFC 2822:

 2.2.1. Unstructured Header Field Bodies

    Some field bodies in this standard are defined simply as
    "unstructured" (which is specified below as any US-ASCII characters,
    except for CR and LF) with no further restrictions.  These are
    referred to as unstructured field bodies.  Semantically, unstructured
    field bodies are simply to be treated as a single line of characters
    with no further processing (except for header "folding" and
    "unfolding" as described in section 2.2.3).