🐛 Fix 2 issues with custom html files in export

1. While exporting, if a route such as `[slug].html.js` is made that serves it's own html pages(with correct Content-Type header), those pages url's were being rewritten even though they were valid. Example: `simple.html` was being rewritten to `simple.html/index.html`. We now check for a `.html` on the end of the path before adding a `/index.html` to the end
2. When parsing exported html, the `<base>` element was checked for in the head of the doc. If this element didn't exist(for example if you generate your own html and miss adding this element) the error message is not clear what is wrong and is hard to track down. This change should avoid that confusion and use a sane default if not available.

Author: jamesjnadeau

src/api/export.ts

@@ -112,7 +112,8 @@ async function _export({
 		const is_html = type === 'text/html';
 
 		if (is_html) {
-			if (pathname !== '/service-worker-index.html') {
+			if (pathname !== '/service-worker-index.html'
+			   && pathname.substr(pathname.length - 5) !== '.html')) {
 				file = file === '' ? 'index.html' : `${file}/index.html`;
 			}
 			body = minify_html(body);
@@ -186,7 +187,7 @@ async function _export({
 					const cleaned = clean_html(body);
 
 					const base_match = /<base ([\s\S]+?)>/m.exec(cleaned);
-					const base_href = base_match && get_href(base_match[1]);
+					 base_match ? get_href(base_match[1]) : '/'
 					const base = resolve(url.href, base_href);
 
 					let match;