[TIMOB-3145] Using multipart POST with HTTPClient

Need to discuss how we're going to break up XHR/HTTPClient going forward. Don't want to put multipart features in a single-request architecture.

Call me naive if you want, but what about keeping full compatibility with the current API by using an optional second parameter for HTTPClient.send? I was thinking in something like:

   HTTPClient.send(data, multipart)
   

where data would have the current semantics and multipart, if present, would be an Array with multiple file objects and their properties:

   {
      { name :'fieldname',
        filename :'filename',
        filedata :'filedata'
        [, 'Content-Type' : 'content-type']
        [, 'Content-Transfer-Encoding' : 'encoding']
      },
      { ... },
      ...
   }
   

The presence of multipart would also signal the need for a multipart protocol instead of the default one. In that case the function would change the response preamble accordingly and add a functional boundary string.

The function could also infer Content-Type for each part using the filename extension, so a full example could be something like:

   var avatardata = avatarfile.read();
   var videodata = videofile.read();
   
   var data = {name : 'a name', option : 'some value'};
   var multipart = {
      { name = 'avatar', filename = 'thumb.jpg',
        filedata =  avatardata,
        Content-Transfer-Encoding' : 'base64' },
      { name = 'video',
        filename = 'presentation.avi',
        filedata = videodata }
   };
   
   HTTPClient.send(data, multipart);