Description:
------------
The specification outlined at http://json.org/ indicates that the empty 
string is a perfectly valid key in a JSON data structure.  JSON 
implementations in Python and Ruby agree.  PHP appears to go out of its 
way to handle the empty string as as key in a non-standard way.

Reproduce code:
---------------
$ php -r 'echo json_encode(json_decode("{\"\":\"foo\"}")), "\n";'
{"_empty_":"foo"}
$

Expected result:
----------------
I expect to see exactly the same output from PHP as seen from Ruby and 
Python:

$ ruby -rjson -e 'puts JSON.parse("{\"\":\"foo\"}").to_json'
{"":"foo"}
$ python -c 'import json; 
print(json.dumps(json.loads("{\"\":\"foo\"}")))'
{"": "foo"}
$

Actual result:
--------------
$ php -r 'echo json_encode(json_decode("{\"\":\"foo\"}")), "\n";'
{"_empty_":"foo"}
$