handle mixed multipart

ex:

    1> hackney:start().
    ok
    2> {ok, S, H, R} =
hackney:get(<<"http://localhost:5984/testdb/bc643432598834a0c669275905000047?open_revs=all">>,
[{<<"Accept">>, <<"multipart/mixed">>}], <<>>, []).
    {ok,200,
        [{<<"Transfer-Encoding">>,<<"chunked">>},
         {<<"Server">>,
          <<"CouchDB/1.3.0 Rcouch/rcouch-0.7.2-4-g2f8c3b3 (Erlang
OTP/R16B02)">>},
         {<<"Date">>,<<"Tue, 17 Dec 2013 22:46:25 GMT">>},
         {<<"Content-Type">>,
          <<"multipart/mixed;
boundary=\"bb124fea1f12a13f9d575ba5ec35642b\"">>}],
        #Ref<0.0.0.91>}
    3>
    3> hackney:stream_multipart(R).
    mp_mixed
    4> hackney:stream_multipart(R).
    {headers,[{<<"Content-Type">>,<<"application/json">>}]}
    5> hackney:stream_multipart(R).
    {body,<<"{\"_id\":\"bc643432598834a0c669275905000047\",\"_rev\":\"2-7fc38de4a78235b650ca135ad637e268\",\"_attachments\":{\"lt232"...>>}
    6> hackney:stream_multipart(R).
    end_of_part
    7> hackney:stream_multipart(R).
    {headers,[]}
    8> hackney:stream_multipart(R).
    {body,<<0,0,27,72,87,71,84,73,2,16,0,0,109,110,116,114,
            82,71,66,32,88,89,90,32,7,219,0,...>>}
    9> hackney:stream_multipart(R).
    end_of_part
    10> hackney:stream_multipart(R).
    mp_mixed_eof
    11> hackney:stream_multipart(R).
    end_of_part
    12> hackney:stream_multipart(R).
    eof