* ext/bigdecimal/bigdecimal.c (BigMath_s_exp): Fix for the cases when
  the argument x is not a BigDecimal.
  This change is based on the patch made by Garth Snyder.
  [Fix GH-332] https://github.com/ruby/ruby/pull/332

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41623 b2dd03c8-39d4-4d8f-98ff-823fe69b080e