fix: Now, setting encode_escape_forward_slash on a cjson instance does not affect other instances.

[SnowOnion]

The new test case "Set encode_escape_forward_slash on a cjson instance should not affect other instances" passes with the fix, and fails without the fix.

[SnowOnion]

🟢New function code + new test:

➜  tests git:(fix-encode_escape_forward_slash) ✗ pwd
/root/develop/github.com/openresty/lua-cjson/tests

➜  tests git:(fix-encode_escape_forward_slash) ✗ LUA_CPATH="../?.so;;" LUA_PATH="../lua/?.lua;;" luajit ./test.lua
...
==> Test [129] Set encode_escape_forward_slash on a cjson instance should not affect other instances: PASS
[Input] {  }
[Received:success] { "\"\\/\"", "\"/\"", "\"\\/\"" }

🔴Old function code + new test:

➜  tests git:(master) ✗ LUA_CPATH="../?.so;;" LUA_PATH="../lua/?.lua;;" luajit ./test.lua
...
==> Test [129] Set encode_escape_forward_slash on a cjson instance should not affect other instances: FAIL
[Input] {  }
[Expected:success] { "\"\\/\"", "\"/\"", "\"\\/\"" }
[Received:success] { "\"\\/\"", "\"/\"", "\"/\"" }