Fix JSON output so only printable ASCII is produced (seen on Solaris)

 [2016/06/01 02:47:35.149] ======================================================================

 [2016/06/01 02:47:35.149] FAIL: test_jsondump02.test_jsondump02.test_json_all_bytes

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 [2016/06/01 02:47:35.149] Traceback (most recent call last):

 [2016/06/01 02:47:35.149]   File "/data/mci/79c64e8d275bc55f88bcf4f078f80064/wiredtiger/test/suite/test_jsondump02.py", line 380, in test_json_all_bytes

 [2016/06/01 02:47:35.149]     self.check_json(self.table_uri6, table6_json)

 [2016/06/01 02:47:35.149]   File "/data/mci/79c64e8d275bc55f88bcf4f078f80064/wiredtiger/test/suite/test_jsondump02.py", line 75, in check_json

 [2016/06/01 02:47:35.149]     self.assertEqual(v, expect[pos][1])

 [2016/06/01 02:47:35.149] AssertionError: b'\'"value0" : "\\\\u00df\\\\u00e0\\xe1"\' != \'"value0" : "\\\\u00df\\\\u00e0\\\\u00e1"\'\n- "value0" : "\\u00df\\u00e0\xe1"\n?                         ^\n+ "value0" : "\\u00df\\u00e0\\u00e1"\n?                         ^^^^^^\n'

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