User can now use a JsonString as a key or a value

src/ArduinoJson/Strings/FlashStringAdapter.hpp

@@ -0,0 +1,53 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2018
+// MIT License
+
+#pragma once
+
+namespace ARDUINOJSON_NAMESPACE {
+
+class FlashStringAdapter {
+ public:
+  FlashStringAdapter(const __FlashStringHelper* str) : _str(str) {}
+
+  bool equals(const char* expected) const {
+    const char* actual = reinterpret_cast<const char*>(_str);
+    if (!actual || !expected) return actual == expected;
+    return strcmp_P(expected, actual) == 0;
+  }
+
+  bool isNull() const {
+    return !_str;
+  }
+
+  char* save(MemoryPool* pool) const {
+    if (!_str) return NULL;
+    size_t n = size() + 1;  // copy the terminator
+    char* dup = pool->allocFrozenString(n);
+    if (dup) memcpy_P(dup, reinterpret_cast<const char*>(_str), n);
+    return dup;
+  }
+
+  const char* data() const {
+    return 0;
+  }
+
+  size_t size() const {
+    return strlen_P(reinterpret_cast<const char*>(_str));
+  }
+
+  bool isStatic() const {
+    return false;
+  }
+
+ private:
+  const __FlashStringHelper* _str;
+};
+
+inline FlashStringAdapter adaptString(const __FlashStringHelper* str) {
+  return FlashStringAdapter(str);
+}
+
+template <>
+struct IsString<const __FlashStringHelper*> : true_type {};
+}  // namespace ARDUINOJSON_NAMESPACE