Allow alternative dict-like objects to be used in parse_dict_header.

RFC 3507 (ICAP) Section 4.4.1 states that the Encapsulated header values
must be ordered monotonically; something the current implementation
doesn't guarantee.

This patch allows the use of an OrderedDict to provide that functionality.
Merge pull request #280 from nathan-hoad/master

Allow alternative dict-like objects to be used in parse_dict_header