[ArduinoJson] Update external library ArduinoJson.

Is needed to compile new ESP32 code.

lib/ArduinoJson/test/DynamicJsonBuffer/startString.cpp

@@ -0,0 +1,47 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2018
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("DynamicJsonBuffer::startString()") {
+  SECTION("WorksWhenBufferIsBigEnough") {
+    DynamicJsonBuffer jsonBuffer(6);
+
+    DynamicJsonBuffer::String str = jsonBuffer.startString();
+    str.append('h');
+    str.append('e');
+    str.append('l');
+    str.append('l');
+    str.append('o');
+
+    REQUIRE(std::string("hello") == str.c_str());
+  }
+
+  SECTION("GrowsWhenBufferIsTooSmall") {
+    DynamicJsonBuffer jsonBuffer(5);
+
+    DynamicJsonBuffer::String str = jsonBuffer.startString();
+    str.append('h');
+    str.append('e');
+    str.append('l');
+    str.append('l');
+    str.append('o');
+
+    REQUIRE(std::string("hello") == str.c_str());
+  }
+
+  SECTION("SizeIncreases") {
+    DynamicJsonBuffer jsonBuffer(5);
+
+    DynamicJsonBuffer::String str = jsonBuffer.startString();
+    REQUIRE(0 == jsonBuffer.size());
+
+    str.append('h');
+    REQUIRE(1 == jsonBuffer.size());
+
+    str.c_str();
+    REQUIRE(2 == jsonBuffer.size());
+  }
+}