Return a structure in postToCollection and putToCollection responses tha...

...t's a little more consistent with what is returned from POSTs to /fs.  The structure that is returned looks like:

{
  document: {
    id: <id>,
    path: <path>,
    uploadDate: <uploadDate>
  }
}

which is similar to the response from a POST to fs:

{
  file: {
    id: <id>,
    :
  }
}

Also fixed what is placed in the Location header.