merge revision(s) 41623,41723: [Backport #6862] [Backport #8587]

	* 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
	  This change is based on the patch made by Heesob Park and Garth Snyder.
	  [Bug #6862] [ruby-core:47145]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@41733 b2dd03c8-39d4-4d8f-98ff-823fe69b080e