call preparePageData on before getting iframe content

lib/single-file/single-file-core.js

@@ -446,6 +446,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						}
 					} else {
 						if (frameData.processor) {
+							await frameData.processor.preparePageData();
 							const pageData = await frameData.processor.getPageData();
 							DomProcessorHelper.setFrameContent(frameElement, pageData.content);
 						}