Add support for object-base returns in API middleware

Previously, GZIP'd content coming through the middleware would immediately error out as it was attempted to convert it to a list

core/src/main/java/org/eclipsefoundation/core/service/impl/DefaultAPIMiddleware.java

 /*********************************************************************
-* Copyright (c) 2019 Eclipse Foundation.
+* Copyright (c) 2019. 2023 Eclipse Foundation.
 *
 * This program and the accompanying materials are made
 * available under the terms of the Eclipse Public License 2.0
@@ -11,8 +11,14 @@
 **********************************************************************/
 package org.eclipsefoundation.core.service.impl;
 
+import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.io.StringWriter;
+import java.io.Writer;
 import java.net.URI;
+import java.nio.charset.StandardCharsets;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.function.Function;
@@ -100,7 +106,13 @@ public class DefaultAPIMiddleware implements APIMiddleware {
         try {
             if (o instanceof InputStream
                     && (r.getHeaderString("Content-Encoding") != null && r.getHeaderString("Content-Encoding").equalsIgnoreCase("GZIP"))) {
-                out.addAll(objectMapper.readerForListOf(type).readValue(new GZIPInputStream((InputStream) o)));
+                // extract content so we can determine whether its a list or single object
+                String content = convertToString(new GZIPInputStream((InputStream) o));
+                if (content.startsWith("[")) {
+                    out.addAll(objectMapper.readerForListOf(type).readValue(content));
+                } else {
+                    out.add(objectMapper.readerFor(type).readValue(content));
+                }
             } else if (o instanceof InputStream) {
                 out.addAll(objectMapper.readerForListOf(type).readValue((InputStream) o));
             } else if (type.isAssignableFrom(o.getClass())) {
@@ -114,6 +126,23 @@ public class DefaultAPIMiddleware implements APIMiddleware {
         return out;
     }
 
+    /**
+     * Converts UTF-8 GZIP'd content to a String to be later analyzed and consumed.
+     * 
+     * @param gzipInputStream the GZIP'd content
+     * @return a string containing the uncompressed data
+     * @throws IOException if there is an error reading the GZIP stream
+     */
+    private String convertToString(GZIPInputStream gzipInputStream) throws IOException {
+        try (Reader reader = new InputStreamReader(gzipInputStream, StandardCharsets.UTF_8); Writer writer = new StringWriter()) {
+            char[] buffer = new char[10240];
+            for (int length = 0; (length = reader.read(buffer)) > 0;) {
+                writer.write(buffer, 0, length);
+            }
+            return writer.toString();
+        }
+    }
+
     private String getURISafely(URI uri) {
         if (uri == null) {
             return "";