FIX Unquoted shortcodes weren't parsed (fixes #680)

Since that used to be the default shortcode notation
for our core "insert media" functionality, its important
to have this fixed and keep supporting "legacy" content
created with 3.0.
Merge pull request #1804 from chillu/pulls/shortcode-quoting

FIX Unquoted shortcodes weren't parsed (fixes #680)