Add misc:json_encode_With_kv_lists and use it in matrix sign function

R27 json module doesn't recognize list of tuples as object specification,
so this creates wrapper that offer this functionality.

src/misc.erl

@@ -42,7 +42,7 @@
 	 is_mucsub_message/1, best_match/2, pmap/2, peach/2, format_exception/4,
 	 get_my_ipv4_address/0, get_my_ipv6_address/0, parse_ip_mask/1,
 	 crypto_hmac/3, crypto_hmac/4, uri_parse/1, uri_parse/2, uri_quote/1,
-         json_encode/1, json_decode/1,
+         json_encode/1, json_decode/1, json_encode_with_kv_lists/1,
 	 set_proc_label/1,
 	 match_ip_mask/3, format_hosts_list/1, format_cycle/1, delete_dir/1,
 	 semver_to_xxyy/1, logical_processors/0, get_mucsub_event_type/1]).
@@ -132,11 +132,20 @@ crypto_hmac(Type, Key, Data, MacL) -> crypto:macN(hmac, Type, Key, Data, MacL).
 -endif.
 
 -ifdef(OTP_BELOW_27).
+json_encode_with_kv_lists(Term) ->
+    jiffy:encode(Term).
 json_encode(Term) ->
     jiffy:encode(Term).
 json_decode(Bin) ->
     jiffy:decode(Bin, [return_maps]).
 -else.
+json_encode_with_kv_lists(Term) ->
+    iolist_to_binary(json:encode(Term),
+		     fun([{_, _} | _] = Val, Encoder) ->
+			 json:encode_key_value_list(Val, Encoder);
+			(Val, Encoder) ->
+			    json:encode_value(Val, Encoder)
+		     end).
 json_encode(Term) ->
     iolist_to_binary(json:encode(Term)).
 json_decode(Bin) ->

src/mod_matrix_gw.erl

@@ -673,7 +673,7 @@ get_pruned_event_id(PrunedEvent) ->
 
 encode_canonical_json(JSON) ->
     JSON2 = sort_json(JSON),
-    misc:json_encode(JSON2).
+    misc:json_encode_with_kv_lists(JSON2).
 
 sort_json(#{} = Map) ->
     Map2 = maps:map(fun(_K, V) ->