fix: Adding response media type is breaking change (#328)

Adding a response media type is a non-breaking change, as long as preexisting ones are still compatible. Clients still can requests all existing media type via `Accept` header as they have done before.

Removing an existing media type on the over hand is a breaking change, because existing client's requesting this response representation (again via `Accept` header) will probably get a `406 Not Acceptable`.

Author: slinstaedt

core/src/main/java/org/openapitools/openapidiff/core/model/ChangedContent.java

@@ -31,7 +31,7 @@ public DiffResult isCoreChanged() {
     if (increased.isEmpty() && missing.isEmpty()) {
       return DiffResult.NO_CHANGES;
     }
-    if (context.isRequest() && missing.isEmpty() || context.isResponse() && increased.isEmpty()) {
+    if (context.isRequest() && missing.isEmpty() || context.isResponse() && missing.isEmpty()) {
       return DiffResult.COMPATIBLE;
     }
     return DiffResult.INCOMPATIBLE;

core/src/test/java/org/openapitools/openapidiff/core/ContentDiffTest.java

@@ -33,15 +33,15 @@ public void testAddedResponseContentTypeDiff() {
     ChangedOpenApi changedOpenApi =
         OpenApiCompare.fromLocations(
             "content_type_response_add_1.yaml", "content_type_response_add_2.yaml");
-    assertThat(changedOpenApi.isCompatible()).isFalse();
+    assertThat(changedOpenApi.isCompatible()).isTrue();
   }
 
   @Test
   public void testRemovedResponseContentTypeDiff() {
     ChangedOpenApi changedOpenApi =
         OpenApiCompare.fromLocations(
             "content_type_response_add_2.yaml", "content_type_response_add_1.yaml");
-    assertThat(changedOpenApi.isCompatible()).isTrue();
+    assertThat(changedOpenApi.isCompatible()).isFalse();
   }
 
   @Test