Allow JSON::Fragment as a return value for as_json

Instead of overriding to_json to return a piece of JSON to be directly
embedded in the JSON document, we can now return a JSON::Fragment from
as_json.

Since both JSON::Coder and JSON::Fragment were shipped together in json
2.10.0, I didn't update the defined? guards.

Co-authored-by: Jean Boussier <[email protected]>

Author: etiennebarrie

activesupport/lib/active_support/json/encoding.rb

@@ -112,7 +112,7 @@ def stringify(jsonified)
 
       if defined?(::JSON::Coder)
         class JSONGemCoderEncoder # :nodoc:
-          JSON_NATIVE_TYPES = [Hash, Array, Float, String, Symbol, Integer, NilClass, TrueClass, FalseClass].freeze
+          JSON_NATIVE_TYPES = [Hash, Array, Float, String, Symbol, Integer, NilClass, TrueClass, FalseClass, ::JSON::Fragment].freeze
           CODER = ::JSON::Coder.new do |value|
             json_value = value.as_json
             # Handle objects returning self from as_json

activesupport/test/json/encoding_test_cases.rb

@@ -56,6 +56,16 @@ def to_json(options = nil)
     end
   end
 
+  class CustomNumericFixed < Numeric
+    def initialize(str)
+      @str = str
+    end
+
+    def as_json
+      ::JSON::Fragment.new(@str)
+    end
+  end
+
   module EncodingTestCases
     TrueTests     = [[ true,  %(true)  ]]
     FalseTests    = [[ false, %(false) ]]
@@ -68,7 +78,8 @@ module EncodingTestCases
                      [ BigDecimal("0.0") / BigDecimal("0.0"),  %(null) ],
                      [ BigDecimal("2.5"), %("#{BigDecimal('2.5')}") ],
                      [ RomanNumeral.new("MCCCXXXVII"), %("MCCCXXXVII") ],
-                     [ [CustomNumeric.new("123")], %([123]) ]
+                     [ [CustomNumeric.new("123")], %([123]) ],
+                     [ [CustomNumericFixed.new("123")], %([123]) ],
     ]
 
     StringTests   = [[ "this is the <string>",     %("this is the \\u003cstring\\u003e")],