Fix test output for JSON

Failure was obscured by other JSON output failures under Cygwin.

test/data/ListCommandTest.json

-[{"completed": false, "completion_date": null, "contexts": ["Context2"], "creation_date": null, "priority": "C", "projects": ["Project1"], "source": "(C) 2015-11-05 Foo @Context2 Not@Context +Project1 Not+Project", "tags": [], "text": "Foo @Context2 Not@Context +Project1 Not+Project"}, {"completed": false, "completion_date": null, "contexts": [], "creation_date": null, "priority": "C", "projects": [], "source": "(C) Drink beer @ home", "tags": [], "text": "Drink beer @ home"}, {"completed": false, "completion_date": null, "contexts": [], "creation_date": null, "priority": "C", "projects": [], "source": "(C) 13 + 29 = 42", "tags": [], "text": "13 + 29 = 42"}, {"completed": false, "completion_date": null, "contexts": ["Context1"], "creation_date": null, "priority": "D", "projects": ["Project2"], "source": "(D) Bar @Context1 +Project2 p:1", "tags": [["p", "1"]], "text": "Bar @Context1 +Project2"}]
+[{"completed": false, "completion_date": null, "contexts": ["Context2"], "creation_date": "2015-11-05", "priority": "C", "projects": ["Project1"], "source": "(C) 2015-11-05 Foo @Context2 Not@Context +Project1 Not+Project", "tags": [], "text": "Foo @Context2 Not@Context +Project1 Not+Project"}, {"completed": false, "completion_date": null, "contexts": [], "creation_date": null, "priority": "C", "projects": [], "source": "(C) Drink beer @ home", "tags": [], "text": "Drink beer @ home"}, {"completed": false, "completion_date": null, "contexts": [], "creation_date": null, "priority": "C", "projects": [], "source": "(C) 13 + 29 = 42", "tags": [], "text": "13 + 29 = 42"}, {"completed": false, "completion_date": null, "contexts": ["Context1"], "creation_date": null, "priority": "D", "projects": ["Project2"], "source": "(D) Bar @Context1 +Project2 p:1", "tags": [["p", "1"]], "text": "Bar @Context1 +Project2"}]