fix: invert priority-data check in HEADERS frame encoder

The condition guarding HEADERS priority encoding was backwards:
it checked is_falsy(StreamDep) (true when undefined) instead
of not is_falsy(StreamDep) (true when present). So priority
data was silently dropped from every HEADERS frame that carried
it, while the PRIORITY flag was never set.

Found by adding HEADERS property tests to prop_gen_http_parser_h2.

Author: codeadict

src/gen_http_parser_h2.erl

@@ -35,14 +35,14 @@ encode(
     } = _Frame
 ) ->
     {Payload, Flags1} =
-        case is_falsy(StreamDep) andalso is_falsy(Weight) andalso is_boolean(IsExclusive) of
+        case (not is_falsy(StreamDep)) andalso (not is_falsy(Weight)) of
             true ->
-                IsExclussiveInt = boolean_to_integer(IsExclusive),
+                ExclusiveBit = boolean_to_integer(IsExclusive =:= true),
                 {
-                    [<<IsExclussiveInt:1, StreamDep:31>>, Weight - 1, Hbf],
+                    [<<ExclusiveBit:1, StreamDep:31>>, Weight - 1, Hbf],
                     set_flags(Flags, headers, [priority])
                 };
-            _Other ->
+            false ->
                 {Hbf, Flags}
         end,