fix: Use ContentType parser in KafkaBinding#probable_event?

Replace raw string comparison with ContentType parsing so that
mixed-case content-type values (e.g. Application/CloudEvents+JSON)
are correctly detected as CloudEvents.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Daniel Azuma <dazuma@gmail.com>

Author: dazuma

lib/cloud_events/kafka_binding.rb

@@ -159,9 +159,10 @@ def register_formatter_methods(formatter,
     def probable_event?(message)
       headers = message[:headers] || {}
       return true if headers.key?("ce_specversion")
-      content_type = headers["content-type"]
-      return false unless content_type
-      content_type.start_with?("application/cloudevents")
+      content_type_string = headers["content-type"]
+      return false unless content_type_string
+      content_type = ContentType.new(content_type_string)
+      content_type.media_type == "application" && content_type.subtype_base == "cloudevents"
     end
 
     ##

test/test_kafka_binding.rb

@@ -79,6 +79,15 @@
       refute kafka_binding.probable_event?(message)
     end
 
+    it "detects a probable structured event with mixed-case content type" do
+      message = {
+        key: nil,
+        value: "{}",
+        headers: { "content-type" => "Application/CloudEvents+JSON" },
+      }
+      assert kafka_binding.probable_event?(message)
+    end
+
     it "returns false for a message with no relevant headers" do
       message = {
         key: nil,