Fix breaking changes in Render contract

Author: DrSatyr

core/src/main/java/org/openapitools/openapidiff/core/output/Render.java

@@ -1,13 +1,21 @@
 package org.openapitools.openapidiff.core.output;
 
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.OutputStreamWriter;
 import org.openapitools.openapidiff.core.exception.RendererException;
 import org.openapitools.openapidiff.core.model.ChangedOpenApi;
 
 public interface Render {
 
-  void render(ChangedOpenApi diff, OutputStreamWriter outputStreamWriter);
+  void render(ChangedOpenApi diff, OutputStreamWriter outputStreamWriter) throws RendererException;
+
+  default String render(ChangedOpenApi diff) throws RendererException {
+    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(new ByteArrayOutputStream());
+    render(diff, outputStreamWriter);
+
+    return outputStreamWriter.toString();
+  }
 
   default void safelyAppend(OutputStreamWriter outputStreamWriter, String csq) {
     try {