Make BigDecimal casting consistent on different platforms

Right now it behaves differently on JRuby:

```
--- expected
+++ actual
@@ -1 +1 @@
-#<BigDecimal:5f3c866c,'0.333333333333333333',18(20)>
+#<BigDecimal:16e0afab,'0.3333333333333333',16(20)>
```

My initial PR (https://github.com/rails/rails/pull/27324)
offered to let the precision to be decided by the platform and
change the test expection, but other contributors suggested
that we should change the default precision in Rails
to be consistent of all platforms.

The value (18) comes from the max default precision that comes
from casting Rational(1/3) to BigDecimal.

activemodel/lib/active_model/type/decimal.rb

@@ -4,6 +4,7 @@ module ActiveModel
   module Type
     class Decimal < Value # :nodoc:
       include Helpers::Numeric
+      BIGDECIMAL_PRECISION = 18
 
       def type
         :decimal
@@ -21,7 +22,7 @@ module ActiveModel
             when ::Float
               convert_float_to_big_decimal(value)
             when ::Numeric, ::String
-              BigDecimal(value, precision.to_i)
+              BigDecimal(value, precision || BIGDECIMAL_PRECISION)
             else
               if value.respond_to?(:to_d)
                 value.to_d