Lazily encode data, params, files

Lazily encode data, params, files

Previously, data, params, and files were encoded and stored in
Request.__init__, and subsequently put into service during
Request.send. The problem with this approach is that hooks and auth
callables need to be aware of the eager encoding, and if they touch the
originals, make sure to update the encoded versions.

A better approach is to only encode late in the sending process. This
way, hooks and auth callables can safely make changes without fear of
the old, encoded variant overriding it.