Rename library/bigdecimal/new_spec.rb -> BigDecimal_spec.rb

Modified describe text from `BigDecimal#new` to `Kernel#BigDecimal`

Author: yhirano55

library/bigdecimal/new_spec.rb renamed to library/bigdecimal/BigDecimal_spec.rb

@@ -1,7 +1,7 @@
 require_relative '../../spec_helper'
 require 'bigdecimal'
 
-describe "BigDecimal.new" do
+describe "Kernel#BigDecimal" do
 
   it "creates a new object of class BigDecimal" do
     BigDecimal("3.14159").should be_kind_of(BigDecimal)