Encode json output before writing test data

[vbisserie]

Signed-off-by: Vincent Bisserie vincent.bisserie@renault.com
Running python test_rule.py --days 1 --save-json bar.json ./rules/bar.yaml raises the following exception :

Traceback (most recent call last):
  File "/home/foo/elastalert/test_rule.py", line 455, in main
    test_instance.run_rule_test()
  File "/home/foo/elastalert/test_rule.py", line 437, in run_rule_test
    data_file.write(json.dumps([doc['_source'] for doc in hits], indent=4))
TypeError: a bytes-like object is required, not 'str'