Make HTTP Header class handle multiple cache-control fields.

The HTTP specification states the following about the fields:
Multiple message-header fields with the same field-name MAY be present
in a message if and only if the entire field-value for that header field
is defined as a comma-separated list [i.e., #(values)]. It MUST be
possible to combine the multiple header fields into one "field-name:
field-value" pair, without changing the semantics of the message, by
appending each subsequent field-value to the first, each separated by a
comma. The order in which header fields with the same field-name are
received is therefore significant to the interpretation of the combined
field value, and thus a proxy MUST NOT change the order of these field
values when a message is forwarded.

Change-Id: I1a6fe5cc8f541f8e80d559641d270d09eac9d85c