Suppress `warning: BigDecimal.new is deprecated` in Active Model

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

Refer ruby/bigdecimal@5337373

* This commit has been made as follows:

```ruby
$ cd activemodel/
$ git grep -l BigDecimal.new | grep \.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```

* This commit has been tested with these Ruby versions:

```
ruby 2.5.0dev (2017-12-15 trunk 61262) [x86_64-linux]
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
ruby 2.3.5p376 (2017-09-14 revision 59905) [x86_64-linux]
ruby 2.2.8p477 (2017-09-14 revision 59906) [x86_64-linux]
```

1 files changed, 8 insertions, 8 deletions

diff --git a/activemodel/test/cases/type/decimal_test.rb b/activemodel/test/cases/type/decimal_test.rb
index b91d67f95f..c0cf6ce590 100644
--- a/activemodel/test/cases/type/decimal_test.rb
+++ b/activemodel/test/cases/type/decimal_test.rb
@@ -7,22 +7,22 @@ module ActiveModel
     class DecimalTest < ActiveModel::TestCase
       def test_type_cast_decimal
         type = Decimal.new
-        assert_equal BigDecimal.new("0"), type.cast(BigDecimal.new("0"))
-        assert_equal BigDecimal.new("123"), type.cast(123.0)
-        assert_equal BigDecimal.new("1"), type.cast(:"1")
+        assert_equal BigDecimal("0"), type.cast(BigDecimal("0"))
+        assert_equal BigDecimal("123"), type.cast(123.0)
+        assert_equal BigDecimal("1"), type.cast(:"1")
       end
 
       def test_type_cast_decimal_from_invalid_string
         type = Decimal.new
         assert_nil type.cast("")
-        assert_equal BigDecimal.new("1"), type.cast("1ignore")
-        assert_equal BigDecimal.new("0"), type.cast("bad1")
-        assert_equal BigDecimal.new("0"), type.cast("bad")
+        assert_equal BigDecimal("1"), type.cast("1ignore")
+        assert_equal BigDecimal("0"), type.cast("bad1")
+        assert_equal BigDecimal("0"), type.cast("bad")
       end
 
       def test_type_cast_decimal_from_float_with_large_precision
         type = Decimal.new(precision: ::Float::DIG + 2)
-        assert_equal BigDecimal.new("123.0"), type.cast(123.0)
+        assert_equal BigDecimal("123.0"), type.cast(123.0)
       end
 
       def test_type_cast_from_float_with_unspecified_precision
@@ -48,7 +48,7 @@ module ActiveModel
       def test_type_cast_decimal_from_object_responding_to_d
         value = Object.new
         def value.to_d
-          BigDecimal.new("1")
+          BigDecimal("1")
         end
         type = Decimal.new
         assert_equal BigDecimal("1"), type.cast(value)