fix(dshm/memcached): add a missing return parameter as otherwise pool parameters are not respected

### Summary

@oldium resported this issue in #171.

Author: bungle

lib/resty/session/dshm.lua

@@ -309,7 +309,7 @@ function storage.new(configuration)
   local server_name       = configuration and configuration.server_name
 
   if pool or pool_size or backlog then
-    setmetatable({
+    return setmetatable({
       prefix = prefix,
       suffix = suffix,
       host = host,

lib/resty/session/memcached.lua

@@ -354,7 +354,7 @@ function storage.new(configuration)
   local server_name       = configuration and configuration.server_name
 
   if pool or pool_size or backlog then
-    setmetatable({
+    return setmetatable({
       prefix = prefix,
       suffix = suffix,
       host = host,

spec/01-utils_spec.lua

@@ -136,4 +136,50 @@ describe("Testing utils", function()
       end
     end)
   end)
+  describe("load_storage", function()
+    -- "dshm" is disabled as it currently cannot be checked by CI
+    for _, strategy in ipairs({ "memcached", "mysql", "postgres", "redis" }) do
+      it("respects pool parameters #" .. strategy, function()
+        local storage = assert(utils.load_storage(strategy, {
+          [strategy] = {
+            pool = "doge",
+            pool_size = 10,
+            backlog = 20,
+          },
+        }))
+
+        assert.equal("doge", storage.options.pool)
+        assert.equal(10, storage.options.pool_size)
+        assert.equal(20, storage.options.backlog)
+      end)
+    end
+    it("respects pool parameters #redis-sentinel", function()
+      local storage = assert(utils.load_storage("redis", {
+        redis = {
+          pool = "doge",
+          pool_size = 10,
+          backlog = 20,
+          sentinels = {},
+        },
+      }))
+
+      assert.equal("doge", storage.connector.config.connection_options.pool)
+      assert.equal(10, storage.connector.config.connection_options.pool_size)
+      assert.equal(20, storage.connector.config.connection_options.backlog)
+    end)
+    it("respects pool parameters #redis-cluster", function()
+      local storage = assert(utils.load_storage("redis", {
+        redis = {
+          pool = "doge",
+          pool_size = 10,
+          backlog = 20,
+          nodes = {},
+        },
+      }))
+
+      assert.equal("doge", storage.options.connect_opts.pool)
+      assert.equal(10, storage.options.connect_opts.pool_size)
+      assert.equal(20, storage.options.connect_opts.backlog)
+    end)
+  end)
 end)