Fixed content-type serialization for lists

Author: timocov

http/http-binding/src/main/java/software/amazon/smithy/java/http/binding/PayloadSerializer.java

@@ -94,6 +94,7 @@ public void writeStruct(Schema schema, SerializableStruct struct) {
 
     @Override
     public <T> void writeList(Schema schema, T listState, int size, BiConsumer<T, ShapeSerializer> consumer) {
+        serializer.writePayloadContentType();
         structSerializer.writeList(schema, listState, size, consumer);
     }