function getCopyHeaders(sourceBucket, sourceFilename, headers) {
  sourceFilename = encodeSpecialCharacters(ensureLeadingSlash(sourceFilename));
  headers = utils.merge({}, headers || {});
  headers['x-amz-copy-source'] = '/' + sourceBucket + sourceFilename;
  headers['Content-Length'] = 0; // to avoid Node's automatic chunking if omitted
  return headers;
}
Get headers needed for Client#copy and Client#copyTo.

@param {String} sourceFilename
@param {Object} headers
@api private