[O] org-entitites aren't translated when in LaTeX fragments

[Brice Waegenire]

org-entitites aren't translated when used in a LaTeX fragment.

For example when exporting $a = by(t) \rarr y(t+1) = by^{2}(t)$ to
HTML I get \(a = by(t) \rarr y(t+1) = by^{2}(t)\) whereas I should get
\(a = by(t) \rightarrow y(t+1) = by^{2}(t)\).
\rarr isn't translated to \rightarrow witch make MathJax unhappy:
Undefined control sequence \rarr

Looks like this bug as been reported in 2010
(http://thread.gmane.org/gmane.emacs.orgmode/22628/) but hasn't been
fixed yet.