优化了平衡，尤其对于向后倾倒的几种极限情况进行优化

Shibo-main-TOMMY/lib/ArduinoJson/extras/tests/JsonDocument/swap.cpp

@@ -0,0 +1,27 @@
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+#include <string>
+#include <utility>
+
+using namespace std;
+
+TEST_CASE("std::swap") {
+  SECTION("DynamicJsonDocument*") {
+    DynamicJsonDocument *p1, *p2;
+    swap(p1, p2);  // issue #1678
+  }
+
+  SECTION("DynamicJsonDocument") {
+    DynamicJsonDocument doc1(0x10), doc2(0x20);
+    doc1.set("hello");
+    doc2.set("world");
+
+    swap(doc1, doc2);
+
+    CHECK(doc1.capacity() == 0x20);
+    CHECK(doc1.as<string>() == "world");
+    CHECK(doc2.capacity() == 0x10);
+    CHECK(doc2.as<string>() == "hello");
+  }
+}