Log Message:
Update ruby-mechanize to 2.7.3.

=== 2.7.3

* New Features
  * Allow net-http-persistent instance to be named. #324, John Weir.
  * #save and #save! return filename #340
  * Updated mime-types requirement to 2.x versions.  #348 by Jeff Nyman.

* Bug fix
  * Ensure Download#save! defaults back to original filename if
    none is provided (#300)