Creating folder in 2013 with REST API

Related to question Creating Folder using REST and JavaScript, I used 3 approaches to create folder with REST API (native C++ casablanca lib):

1. 
   

   Works OK

   
   

   
   request.set_request_uri(web::uri::encode_uri(U("/_api/Web/Folders/add('/Shared Documents/new folder created')")));
   
   



2. 
   

   HTTP 400 Bad request (example from MSDN)

   
   

   
   request.set_request_uri(web::uri::encode_uri(U("/_api/Web/GetFolderByServerRelativeUrl('/Shared Documents')/Folders")));
   request.set_body(U("{ '__metadata':{ 'type': 'SP.Folder' }, 'ServerRelativeUrl':'new folder created' }"));
   
   



3. 
   

   HTTP 400 Bad request (example from MSDN)

   
   

   
   request.set_request_uri(web::uri::encode_uri(U("/_api/Web/Folders/")));
   request.set_body(U("{ '__metadata':{ 'type': 'SP.Folder' }, 'ServerRelativeUrl':'Shared Documents/new folder created' }"));
   
   

Are there any ideas why only #1 works?