Allows to encode empty tables as arrays.

An option is added to encode empty Lua tables to arrays. The option is
"encode_empty_table_as_array" and is set like this:

   cjson.encode_empty_table_as_array("on")
   assert( cjson.encode{} == "[]" )

By default, the option is off.

refs #2347
Allows to encode empty tables as arrays.

cherry-pick from https://github.com/ignacio/lua-cjson/commit/a6f950fa3b1f5d2b50a405cca5a74000dcf3dd16