[O] Export as HTML does not preserve nonstandard entities as TODO signs

[Arne Babenhauserheide]

Hi, 

I started using ✔ and ❢ as replacement for DONE und TODO and I realized that 
they get replaced by an underscore (_) in HTML output. Since I really like 
using symbols for TODO and DONE¹, I wanted to ask, if this is a bug. 

Best wishes,
Arne

¹: Just compare: 

* ✔ Sent bug question
* ❢ check for response

and 

* DONE Sent bug question
* TODO check for response

PS: I actually have ✔ on my keyboard, and C-c C-t makes it easy to use any kind 
of symbol for TODO states which is supported by the font.