Fix Data URI encoding problem

The default encoding of the data URI "data:, ..." is US-ASCII and that causes mojibake when UTF-8 percent encoding is used.
So `request.overrideMimeType("text/plain; charset=UTF-8")` is needed.

src/adapt/net.js

@@ -116,6 +116,8 @@ adapt.net.ajax = (url, opt_type, opt_method, opt_data, opt_contentType) => {
                 }
             } else if ((/^data:,(<|%3C|%3c)/).test(url)) {
                 request.overrideMimeType("text/html; charset=UTF-8");
+            } else if ((/^data:,/).test(url)) {
+                request.overrideMimeType("text/plain; charset=UTF-8");
             }
             request.send(null);
         }