fixed a bug when acquiring a connection with a specific id

test/ConnectionPoolTest.cpp

@@ -27,15 +27,24 @@ TEST_CASE("Create connection pool", "[connection pool]") {
   REQUIRE(pool.idle() == 4);
   REQUIRE(pool.inuse() == 0);
 
+  ptr = pool.acquire(3);
+  REQUIRE(ptr.valid());
+  REQUIRE(ptr.id() == 3);
+  REQUIRE(ptr->is_open());
   {
     auto ptr2 = pool.acquire();
     REQUIRE(ptr2.valid());
     REQUIRE(ptr2->is_open());
 
-    REQUIRE(pool.idle() == 3);
-    REQUIRE(pool.inuse() == 1);
+    REQUIRE(pool.idle() == 2);
+    REQUIRE(pool.inuse() == 2);
   }
 
+  REQUIRE(pool.idle() == 3);
+  REQUIRE(pool.inuse() == 1);
+
+  pool.release(ptr);
+  REQUIRE(!ptr.valid());
   REQUIRE(pool.idle() == 4);
   REQUIRE(pool.inuse() == 0);
 }