Fix JSON wrong encoding tests on big endian platforms (#1781)

Fix JSON wrong encoding tests on big endian platforms (#1781)

Fix test_json_without_specified_encoding_*_error tests on big endian
platforms.  The tests wrongly assume that data encoded as "utf-32-be"
can not be decoded as "utf-32".  This is true on little endian platforms
but on big endian platforms "utf-32" is equivalent to "utf-32-be".
To avoid the problem, explicitly decode as "utf-32-le", as this should
trigger the expected exception independently of platform's endianness.