When encoding an array or object ends in an error, the visited flag
wasn't zeroed, causing subsequent encoding attempts to fail. This
patch fixes the problem by always zeroing the visited flag.
Encoding an empty array or object worked, but encoding it again
(possibly after adding some items) failed, because the visited flag
(used for detecting circular references) wasn't zeroed.
* Now that JSON_SORT_KEYS is implemented, take it into use with the
valid and valid-strip suites. This is to ensure that the tests
remain valid even if the string hash function is changed in the
future.
* Remove test_dump API test. Instead, implement the same tests more
elegantly in the encoding-flags suite.
