Support dynamic JSON data content type (#471)

Now checks if `datacontenttype` matches the regex:

`^(application|text)\/([a-zA-Z]+\+)?json$")`

This regex support 
`application/foobar+json`
or standard
```
application/json
text/json
```

Signed-off-by: Isaac Aymerich <isaac.aymerich@roche.com>

formats/json-jackson/src/main/java/io/cloudevents/jackson/JsonFormat.java

@@ -29,6 +29,7 @@ import io.cloudevents.rw.CloudEventDataMapper;
 import io.cloudevents.rw.CloudEventRWException;
 
 import java.io.IOException;
+import java.util.regex.Pattern;
 
 /**
  * Implementation of {@link EventFormat} for <a href="https://github.com/cloudevents/spec/blob/v1.0/json-format.md">JSON event format</a>
@@ -43,7 +44,10 @@ public final class JsonFormat implements EventFormat {
      * Content type associated with the JSON event format
      */
     public static final String CONTENT_TYPE = "application/cloudevents+json";
-
+    /**
+     * JSON Data Content Type Discriminator
+     */
+    private static final Pattern JSON_CONTENT_TYPE_PATTERN = Pattern.compile("^(application|text)\\/([a-zA-Z]+\\+)?json$");
     private final ObjectMapper mapper;
     private final JsonFormatOptions options;
 
@@ -214,6 +218,6 @@ public final class JsonFormat implements EventFormat {
 
     static boolean dataIsJsonContentType(String contentType) {
         // If content type, spec states that we should assume is json
-        return contentType == null || contentType.startsWith("application/json") || contentType.startsWith("text/json");
+        return contentType == null || JSON_CONTENT_TYPE_PATTERN.matcher(contentType).matches();
     }
 }

formats/json-jackson/src/test/java/io/cloudevents/jackson/JsonCloudEventDataTest.java

@@ -23,16 +23,21 @@ import io.cloudevents.core.builder.CloudEventBuilder;
 import io.cloudevents.core.mock.MyCloudEventData;
 import io.cloudevents.core.provider.EventFormatProvider;
 import io.cloudevents.core.test.Data;
-import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import java.util.stream.Stream;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 public class JsonCloudEventDataTest {
 
-    @Test
-    public void testMapper() {
+    @ParameterizedTest
+    @MethodSource("textContentArguments")
+    public void testMapper(String contentType) {
         CloudEvent event = CloudEventBuilder.v1(Data.V1_MIN)
-            .withData("application/json", new JsonCloudEventData(JsonNodeFactory.instance.numberNode(10)))
+            .withData(contentType, new JsonCloudEventData(JsonNodeFactory.instance.numberNode(10)))
             .build();
 
         byte[] serialized = EventFormatProvider.getInstance().resolveFormat(JsonFormat.CONTENT_TYPE)
@@ -47,10 +52,19 @@ public class JsonCloudEventDataTest {
                 return new MyCloudEventData(((JsonCloudEventData) data).getNode().asInt());
             });
 
+        assertThat(deserialized.getDataContentType())
+            .isEqualTo(contentType);
         assertThat(deserialized.getData())
             .isInstanceOf(MyCloudEventData.class);
         assertThat(((MyCloudEventData) deserialized.getData()).getValue())
             .isEqualTo(10);
     }
 
+    public static Stream<Arguments> textContentArguments() {
+        return Stream.of(
+            Arguments.of("application/json"),
+            Arguments.of("text/json"),
+            Arguments.of("application/foobar+json")
+        );
+    }
 }