fix: avoid corrupting the default healthcheck conf (#5)

lib/resty/healthcheck.lua

@@ -154,6 +154,48 @@ local function key_for(key_prefix, ip, port, hostname)
 end
 
 
+local deepcopy
+do
+    local function _deepcopy(orig, copied)
+        -- prevent infinite loop when a field refers its parent
+        copied[orig] = true
+        -- If the array-like table contains nil in the middle,
+        -- the len might be smaller than the expected.
+        -- But it doesn't affect the correctness.
+        local len = #orig
+        local copy = table.new(len, table.nkeys(orig) - len)
+        for orig_key, orig_value in pairs(orig) do
+            if type(orig_value) == "table" and not copied[orig_value] then
+                copy[orig_key] = _deepcopy(orig_value, copied)
+            else
+                copy[orig_key] = orig_value
+            end
+        end
+
+        local mt = getmetatable(orig)
+        if mt ~= nil then
+            setmetatable(copy, mt)
+        end
+
+        return copy
+    end
+
+
+    local copied_recorder = {}
+
+    function deepcopy(orig)
+        local orig_type = type(orig)
+        if orig_type ~= 'table' then
+            return orig
+        end
+
+        local res = _deepcopy(orig, copied_recorder)
+        table.clear(copied_recorder)
+        return res
+    end
+end
+
+
 local checker = {}
 
 
@@ -1203,7 +1245,7 @@ local function fill_in_settings(opts, defaults, ctx)
         obj[k] = v
       end
     elseif default ~= NO_DEFAULT then
-      obj[k] = default
+      obj[k] = deepcopy(default)
     end
 
   end