firebase: add set

Firebase.cpp

@@ -41,6 +41,11 @@ void Firebase::remove(const String& path) {
   sendRequest("DELETE", path);
 }
 
+String Firebase::set(const String& path, const String& value) {
+  sendRequest("PUT", path, value);
+  return readBody();
+}
+
 Firebase& Firebase::stream(const String& path) {
   _error.reset();
   String url = makeURL(path); 

Firebase.h

@@ -51,6 +51,11 @@ class Firebase {
     return _error;
   }
   String get(const String& path);
+  // write a new JSON `value` to the given `path`.
+  // Note: A String `value` must include double quotes to be valid json.
+  String set(const String& path, const String& value);
+  // append a new JSON `value` to the given `path`.
+  // Note: A String `value` must include double quotes to be valid json.
   String push(const String& path, const String& value);
   void remove(const String& path);
   bool connected();