Fix detection of objects with numeric string keys

lua_array_length() recognised some objects with numeric string
keys as arrays since it was incorrectly using lua_isnumber().

When an object was incorrectly recognised as an array,
json_append_array() would not find any entries and generate a
result like:
	[null,null,...]

Reported by: Zhang "agentzh" Yichun <agentzh@gmail.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/tests/test.lua b/tests/test.lua
index 7a9b0ff..7a75243 100755
--- a/tests/test.lua
+++ b/tests/test.lua
@@ -118,13 +118,14 @@ local encode_table_tests = {
     end,
     { json.encode, { { [3] = "sparse test" } },
       true, { '[null,null,"sparse test"]' } },
-
     { json.encode, { { [1] = "one", [4] = "sparse test" } },
       true, { '["one",null,null,"sparse test"]' } },
-
     { json.encode, { { [1] = "one", [5] = "sparse test" } },
       true, { '{"1":"one","5":"sparse test"}' } },
 
+    { json.encode, { { ["2"] = "numeric string key test" } },
+      true, { '{"2":"numeric string key test"}' } },
+
     { json.encode, { nested5 }, true, { '[[[[["nested"]]]]]' } },
     { json.encode, { { nested5 } },
       false, { "Cannot serialise, excessive nesting (6)" } },