Merge pull request #1374 from CiscoM31/set-extension-with-version

Add extension to keep track of original swagger version

Author: gracekarina

modules/swagger-parser-v2-converter/src/main/java/io/swagger/v3/parser/converter/SwaggerConverter.java

@@ -159,6 +159,8 @@ public SwaggerParseResult convert(SwaggerDeserializationResult parse) {
         if (swagger.getVendorExtensions() != null) {
             openAPI.setExtensions(convert(swagger.getVendorExtensions()));
         }
+        // Set extension to retain original version of OAS document.
+        openAPI.addExtension("x-original-swagger-version", swagger.getSwagger());
 
         if (swagger.getExternalDocs() != null) {
             openAPI.setExternalDocs(convert(swagger.getExternalDocs()));