Avro fields with default values, or unions that include the null type…

…, should not be included in required properties of JSON schema

Combined fixes in the following PRs from the forked repository:
fge#2
fge#3

src/main/java/com/github/fge/avro/translators/RecordTranslator.java

@@ -96,7 +96,9 @@ protected void doTranslate(final Schema avroSchema,
             fieldSchema = field.schema();
             fieldType = fieldSchema.getType();
             translator = AvroTranslators.getTranslator(fieldType);
-            required.add(fieldName);
+            if (isRequiredField(field)) {
+                required.add(fieldName);
+            }
             ptr = JsonPointer.of("properties", fieldName);
             propertyNode = FACTORY.objectNode();
             properties.put(fieldName, propertyNode);
@@ -107,6 +109,23 @@ protected void doTranslate(final Schema avroSchema,
         }
     }
 
+    private boolean isRequiredField(Schema.Field field) {
+        if (field.defaultValue() != null) {
+            return false;
+        }
+
+        Schema fieldSchema = field.schema();
+        if (fieldSchema.getType() == Schema.Type.UNION) {
+            for (Schema typeSchema : fieldSchema.getTypes()) {
+                if (typeSchema.getType() == Schema.Type.NULL) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
     private static void injectDefault(final ObjectNode propertyNode,
         final Schema.Field field)
     {