convert all text/* content types to strings in the apache http java client (#18731)

samples/client/echo_api/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java

@@ -777,7 +777,7 @@ public class ApiClient extends JavaTimeFormatter {
       }
 
       return objectMapper.readValue(content, valueType);
-    } else if ("text/plain".equalsIgnoreCase(mimeType)) {
+    } else if (mimeType.toLowerCase().startsWith("text/")) {
       // convert input stream to string
       return (T) EntityUtils.toString(entity);
     } else {