Change IPAddr#to_json to match the behavior of the json gem

Returning the string representation instead of the instance
variables of the object.

    Before:

    ```ruby
    IPAddr.new("127.0.0.1").to_json
    # => "{\"addr\":2130706433,\"family\":2,\"mask_addr\":4294967295}"
    ```

    After:

    ```ruby
    IPAddr.new("127.0.0.1").to_json
    # => "\"127.0.0.1\""
    ```

Fixes #40932.

activesupport/CHANGELOG.md

@@ -1,3 +1,21 @@
+*   Change `IPAddr#to_json` to match the behavior of the json gem returning the string representation
+    instead of the instance variables of the object.
+
+    Before:
+
+    ```ruby
+    IPAddr.new("127.0.0.1").to_json
+    # => "{\"addr\":2130706433,\"family\":2,\"mask_addr\":4294967295}"
+    ```
+
+    After:
+
+    ```ruby
+    IPAddr.new("127.0.0.1").to_json
+    # => "\"127.0.0.1\""
+    ```
+
+
 ## Rails 6.1.0 (December 09, 2020) ##
 
 *   Ensure `MemoryStore` disables compression by default. Reverts behavior of

activesupport/lib/active_support/core_ext/object/json.rb

@@ -3,6 +3,7 @@
 # Hack to load json gem first so we can overwrite its to_json.
 require "json"
 require "bigdecimal"
+require "ipaddr"
 require "uri/generic"
 require "pathname"
 require "active_support/core_ext/big_decimal/conversions" # for #to_s
@@ -219,6 +220,12 @@ def as_json(options = nil)
   end
 end
 
+class IPAddr # :nodoc:
+  def as_json(options = nil)
+    to_s
+  end
+end
+
 class Process::Status #:nodoc:
   def as_json(options = nil)
     { exitstatus: exitstatus, pid: pid }

activesupport/test/json/encoding_test_cases.rb

@@ -86,6 +86,8 @@ module EncodingTestCases
 
     PathnameTests = [[ Pathname.new("lib/index.rb"), %("lib/index.rb") ]]
 
+    IPAddrTests   = [[  IPAddr.new("127.0.0.1"), %("127.0.0.1") ]]
+
     DateTests     = [[ Date.new(2005, 2, 1), %("2005/02/01") ]]
     TimeTests     = [[ Time.utc(2005, 2, 1, 15, 15, 10), %("2005/02/01 15:15:10 +0000") ]]
     DateTimeTests = [[ DateTime.civil(2005, 2, 1, 15, 15, 10), %("2005/02/01 15:15:10 +0000") ]]