Typo in the update_object method.

Hey

It looks like there was a typo in the original EsuApi.rb.  It sets the range header to the string "${extent}".
From the Google code svn, it seems like the original isn't maintained any more.  Thanks for creating the github repo.

In update_object, the range header was set as:
  headers["range"] = "${extent}"
.. it should be:
  headers["range"] = "#{extent}"