[test] avoid BigDecimal.new deprecation on Ruby 2.5

Author: kares

tests/json_parser_test.rb

@@ -110,8 +110,8 @@ def test_parse_numbers
   end
 
   def test_parse_bigdecimals
-    assert_equal(BigDecimal,                                 JSON.parse('{"foo": 9.01234567890123456789}', decimal_class: BigDecimal)["foo"].class)
-    assert_equal(BigDecimal.new("0.901234567890123456789E1"),JSON.parse('{"foo": 9.01234567890123456789}', decimal_class: BigDecimal)["foo"]      )
+    assert_equal(BigDecimal,                             JSON.parse('{"foo": 9.01234567890123456789}', decimal_class: BigDecimal)["foo"].class)
+    assert_equal(BigDecimal("0.901234567890123456789E1"),JSON.parse('{"foo": 9.01234567890123456789}', decimal_class: BigDecimal)["foo"]      )
   end
 
   if Array.method_defined?(:permutation)