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"}
```

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

@@ -2,6 +2,7 @@
 require 'json'
 require 'bigdecimal'
 require 'uri/generic'
+require 'pathname'
 require 'active_support/core_ext/big_decimal/conversions' # for #to_s
 require 'active_support/core_ext/hash/except'
 require 'active_support/core_ext/hash/slice'
@@ -199,6 +200,12 @@ def as_json(options = nil)
   end
 end
 
+class Pathname #: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

@@ -78,6 +78,8 @@ module EncodingTestCases
 
     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") ]]