Always use %f formatter for ISO8601 durations since they can't be in scientific notation

Manually formats the seconds. This avoids using the %g formatter which can convert large numbers or very small fractions into scientific notation which is not supported by iso8601

activesupport/lib/active_support/duration/iso8601_serializer.rb

@@ -27,7 +27,7 @@ module ActiveSupport
         time << "#{parts[:hours]}H"     if parts.key?(:hours)
         time << "#{parts[:minutes]}M"   if parts.key?(:minutes)
         if parts.key?(:seconds)
-          time << "#{sprintf(@precision ? "%0.0#{@precision}f" : '%g', parts[:seconds])}S"
+          time << "#{format_seconds(parts[:seconds])}S"
         end
         output << "T#{time}" unless time.empty?
         output
@@ -54,6 +54,14 @@ module ActiveSupport
         def week_mixed_with_date?(parts)
           parts.key?(:weeks) && (parts.keys & DATE_COMPONENTS).any?
         end
+
+        def format_seconds(seconds)
+          if @precision
+            sprintf("%0.0#{@precision}f", seconds)
+          else
+            seconds.to_s
+          end
+        end
     end
   end
 end

activesupport/test/core_ext/duration_test.rb

@@ -625,6 +625,7 @@ class DurationTest < ActiveSupport::TestCase
       ["P1Y1M1DT1H",    1.year + 1.month + 1.day + 1.hour],
       ["PT0S",          0.minutes                        ],
       ["PT-0.2S",       (-0.2).seconds                   ],
+      ["PT1000000S",    1_000_000.seconds                ],
     ]
     expectations.each do |expected_output, duration|
       assert_equal expected_output, duration.iso8601, expected_output.inspect