Pipelet: ..
  - _add_destination() and _remove_destination(): return this instead of
    destination for consistency with other pipelet methods

  - __add_source() : no-longer allows implicit union injection

  - implement _insert_source_union() to insert explicitly a union as a source
    of a pipelet, switching a previous source if any

  - improve documentation

  - test/http.js: test use of _insert_source_union() along with _add_source()