Convert ASCII spaces to U+2800 characters in 'text_braille' extension
Closes #410 (closed) (see issue for rationale).
I'm not totally sure about this change because this solution converts ASCII spaces to U+2800 characters inside other elements beyond texts. It seems to me that doesn't make sense to use text_braille
extension for other elements beyond texts, but I'm not sure what side effects this change could bring.