Fixes the digits counter of AS's NumberToRoundedConverter

Zero has one digit, but Math.log10(0) returns -Infinity. The method
needs to special-case zero.

The patch adds a regression test that is not clearly related to the
underlying issue because digit_count is private and has no coverage.
Gray area.

This bug was uncovered by 60062cf.

activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb

@@ -59,7 +59,7 @@ module ActiveSupport
         end
 
         def digit_count(number)
-          (Math.log10(absolute_number(number)) + 1).floor
+          number.zero? ? 1 : (Math.log10(absolute_number(number)) + 1).floor
         end
 
         def strip_insignificant_zeros

activesupport/test/number_helper_test.rb

@@ -135,6 +135,7 @@ module ActiveSupport
           assert_equal("111.23460000000000000000", number_helper.number_to_rounded(BigDecimal(111.2346, Float::DIG), :precision => 20))
           assert_equal("111.2346" + "0"*96, number_helper.number_to_rounded('111.2346', :precision => 100))
           assert_equal("111.2346", number_helper.number_to_rounded(Rational(1112346, 10000), :precision => 4))
+          assert_equal('0.00', number_helper.number_to_rounded(Rational(0, 1), :precision => 2))
         end
       end