Customize subsecond digits when encoding DateWithTime

The subsecond fraction digits had been hardcoded to 3. This forced all
timestamps to include the subsecond digits with no way to customize the
value. While the subsecond format is part of the ISO8601 spec, it is not
adhered to by all parsers (notably mobile clients). This adds the
ability to customize the number of digits used, optionally setting them
to 0 in order to eliminate the subsecond fraction entirely:

ActiveSupport::JSON::Encoding.subsecond_fraction_digits = 0

activesupport/lib/active_support/json/encoding.rb

@@ -4,6 +4,7 @@
 module ActiveSupport
   class << self
     delegate :use_standard_json_time_format, :use_standard_json_time_format=,
+      :subsecond_fraction_digits, :subsecond_fraction_digits=,
       :escape_html_entities_in_json, :escape_html_entities_in_json=,
       :encode_big_decimal_as_string, :encode_big_decimal_as_string=,
       :json_encoder, :json_encoder=,
@@ -60,7 +61,7 @@ def to_json(*)
           end
 
           # Mark these as private so we don't leak encoding-specific constructs
-          private_constant :ESCAPED_CHARS, :ESCAPE_REGEX_WITH_HTML_ENTITIES, 
+          private_constant :ESCAPED_CHARS, :ESCAPE_REGEX_WITH_HTML_ENTITIES,
             :ESCAPE_REGEX_WITHOUT_HTML_ENTITIES, :EscapedString
 
           # Convert an object into a "JSON-ready" representation composed of
@@ -105,6 +106,10 @@ class << self
         # as a safety measure.
         attr_accessor :escape_html_entities_in_json
 
+        # Configures the inclusion of subsecond resolution when serializing instances
+        # of ActiveSupport::TimeWithZone.
+        attr_accessor :subsecond_fraction_digits
+
         # Sets the encoder used by Rails to encode Ruby objects into JSON strings
         # in +Object#to_json+ and +ActiveSupport::JSON.encode+.
         attr_accessor :json_encoder

activesupport/lib/active_support/time_with_zone.rb

@@ -154,7 +154,8 @@ def xmlschema(fraction_digits = 0)
     #   # => "2005/02/01 05:15:10 -1000"
     def as_json(options = nil)
       if ActiveSupport::JSON::Encoding.use_standard_json_time_format
-        xmlschema(3)
+        digits = ActiveSupport::JSON::Encoding.subsecond_fraction_digits || 3
+        xmlschema(digits)
       else
         %(#{time.strftime("%Y/%m/%d %H:%M:%S")} #{formatted_offset(false)})
       end

activesupport/test/core_ext/time_with_zone_test.rb

@@ -67,17 +67,25 @@ def test_zone
   end
 
   def test_to_json_with_use_standard_json_time_format_config_set_to_false
-    old, ActiveSupport.use_standard_json_time_format = ActiveSupport.use_standard_json_time_format, false
-    assert_equal "\"1999/12/31 19:00:00 -0500\"", ActiveSupport::JSON.encode(@twz)
-  ensure
-    ActiveSupport.use_standard_json_time_format = old
+    with_standard_json_time_format(false) do
+      assert_equal "\"1999/12/31 19:00:00 -0500\"", ActiveSupport::JSON.encode(@twz)
+    end
   end
 
   def test_to_json_with_use_standard_json_time_format_config_set_to_true
-    old, ActiveSupport.use_standard_json_time_format = ActiveSupport.use_standard_json_time_format, true
-    assert_equal "\"1999-12-31T19:00:00.000-05:00\"", ActiveSupport::JSON.encode(@twz)
+    with_standard_json_time_format(true) do
+      assert_equal "\"1999-12-31T19:00:00.000-05:00\"", ActiveSupport::JSON.encode(@twz)
+    end
+  end
+
+  def test_to_json_with_custom_subsecond_resolution
+    with_standard_json_time_format(true) do
+      ActiveSupport::JSON::Encoding.subsecond_fraction_digits = 0
+
+      assert_equal "\"1999-12-31T19:00:00-05:00\"", ActiveSupport::JSON.encode(@twz)
+    end
   ensure
-    ActiveSupport.use_standard_json_time_format = old
+    ActiveSupport::JSON::Encoding.subsecond_fraction_digits = nil
   end
 
   def test_to_json_when_wrapping_a_date_time
@@ -814,6 +822,13 @@ def with_env_tz(new_tz = 'US/Eastern')
     ensure
       old_tz ? ENV['TZ'] = old_tz : ENV.delete('TZ')
     end
+
+    def with_standard_json_time_format(boolean = true)
+      old, ActiveSupport.use_standard_json_time_format = ActiveSupport.use_standard_json_time_format, boolean
+      yield
+    ensure
+      ActiveSupport.use_standard_json_time_format = old
+    end
 end
 
 class TimeWithZoneMethodsForTimeAndDateTimeTest < ActiveSupport::TestCase