Suppress `warning: BigDecimal.new is deprecated`

`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
 which will be a default for Ruby 2.5.

Refer
https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503

* This commit has been made as follows:

```
cd rails
git grep -l BigDecimal.new | grep -v guides/source/5_0_release_notes.md | grep -v activesupport/test/xml_mini_test.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```
- `activesupport/test/xml_mini_test.rb`
Editmanually to remove `.new` and `::`

- guides/source/5_0_release_notes.md
This is a Rails 5.0 release notes.

1 files changed, 1 insertions, 1 deletions

diff --git a/activesupport/test/xml_mini_test.rb b/activesupport/test/xml_mini_test.rb
index 5e46406a7b..18a3f2ca66 100644
--- a/activesupport/test/xml_mini_test.rb
+++ b/activesupport/test/xml_mini_test.rb
@@ -114,7 +114,7 @@ module XmlMiniTest
     end
 
     test "#to_tag accepts decimal types" do
-      @xml.to_tag(:b, ::BigDecimal.new("1.2"), @options)
+      @xml.to_tag(:b, BigDecimal("1.2"), @options)
       assert_xml("<b type=\"decimal\">1.2</b>")
     end