* ext/bigdecimal/bigdecimal.c (cannot_be_coerced_into_BigDecimal):
      add a new function for raising error when an object cannot coerce
      into BigDecimal.  [Bug #5172]
    * ext/bigdecimal/bigdecimal.c (BigDecimalValueWithPrec): use
      cannot_be_coerced_into_BigDecimal function.
    * ext/bigdecimal/bigdecimal.c (BigMath_s_exp): ditto.
    * ext/bigdecimal/bigdecimal.c (BigMath_s_log): ditto.
    * test/bigdecimal/test_bigdecimal.rb: test for the avobe changes.
    * test/bigdecimal/testbase.rb (under_gc_stress): add a new utility
      method to run tests under the condition of GC.stress = true.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@32997 b2dd03c8-39d4-4d8f-98ff-823fe69b080e