Fix int definition

Author: Yuki Ito

src/msgpack_packer.erl

@@ -98,13 +98,13 @@ pack(Other, _) ->
 pack_int(N) when N >= -32->
     << 2#111:3, N:5 >>;
 %% int 8
-pack_int(N) when N > -128 ->
+pack_int(N) when N >= -128 ->
     << 16#D0:8, N:8/big-signed-integer-unit:1 >>;
 %% int 16
-pack_int(N) when N > -32768 ->
+pack_int(N) when N >= -16#8000 ->
     << 16#D1:8, N:16/big-signed-integer-unit:1 >>;
 %% int 32
-pack_int(N) when (N band 16#FFFFFFFF) =:= N ->
+pack_int(N) when N >= -16#80000000 ->
     << 16#D2:8, N:32/big-signed-integer-unit:1 >>;
 %% int 64
 pack_int(N) when N >= -16#8000000000000000 ->