zjsquery: Give special treatment to "body" tag.

We don't do things like $('table') much in our code, but $('body')
is in the code.

frontend_tests/zjsunit/zjquery.js

@@ -315,6 +315,7 @@ exports.make_zjquery = function () {
             ('<#.'.indexOf(selector[0]) >= 0) ||
             (selector === 'window-stub') ||
             (selector === 'document-stub') ||
+            (selector === 'body') ||
             (selector === 'html') ||
             (selector.location) ||
             (selector.indexOf('#') >= 0) ||