Define `Pathname#as_json`

When the Pathname object is converted as JSON,
it should be a string that means itself.

Expected:
```
>> Pathname.new('/path/to/somewhere.txt').as_json
"/path/to/somewhere.txt"
```

Actual:
```
>> Pathname.new('/path/to/somewhere.txt').as_json
{"path"=>"/path/to/somewhere.txt"}
```

1 files changed, 2 insertions, 0 deletions

diff --git a/activesupport/test/json/encoding_test_cases.rb b/activesupport/test/json/encoding_test_cases.rb
index 60d8685d32..e043fadf56 100644
--- a/activesupport/test/json/encoding_test_cases.rb
+++ b/activesupport/test/json/encoding_test_cases.rb
@@ -78,6 +78,8 @@ module JSONTest
 
     URITests      = [[ URI.parse('http://example.com'), %("http://example.com") ]]
 
+    PathnameTests = [[ Pathname.new('lib/index.rb'), %("lib/index.rb") ]]
+
     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") ]]