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, 3 insertions, 3 deletions

diff --git a/actionview/test/template/form_helper_test.rb b/actionview/test/template/form_helper_test.rb
index e7b249cc9c..6a317e1a12 100644
--- a/actionview/test/template/form_helper_test.rb
+++ b/actionview/test/template/form_helper_test.rb
@@ -747,19 +747,19 @@ class FormHelperTest < ActionView::TestCase
   end
 
   def test_check_box_with_explicit_checked_and_unchecked_values_when_object_value_is_big_decimal
-    @post.secret = BigDecimal.new(0)
+    @post.secret = BigDecimal(0)
     assert_dom_equal(
       '<input name="post[secret]" type="hidden" value="1" /><input checked="checked" id="post_secret" name="post[secret]" type="checkbox" value="0" />',
       check_box("post", "secret", {}, 0, 1)
     )
 
-    @post.secret = BigDecimal.new(1)
+    @post.secret = BigDecimal(1)
     assert_dom_equal(
       '<input name="post[secret]" type="hidden" value="1" /><input id="post_secret" name="post[secret]" type="checkbox" value="0" />',
       check_box("post", "secret", {}, 0, 1)
     )
 
-    @post.secret = BigDecimal.new(2.2, 1)
+    @post.secret = BigDecimal(2.2, 1)
     assert_dom_equal(
       '<input name="post[secret]" type="hidden" value="1" /><input id="post_secret" name="post[secret]" type="checkbox" value="0" />',
       check_box("post", "secret", {}, 0, 1)