= http: fix Cache-Control directives parser, fixes #862

3 changes:
  - `no-cache = ...` never worked because of ordering of parsing alternatives
  - list of field-names must be quoted completely not per-element
  - allow `token` for `no-cache` / `private` field-names to be more lenient
    even if the httpbis specification says

    "This directive uses the quoted-string form of the argument syntax.  A
     sender SHOULD NOT generate the token form (even if quoting appears
     not to be needed for single-entry lists)."

     (http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-26#section-5.2.2.2)