Update use of open() in object API

* Switch to use io.open() for py3 compatibility and simpler testing.
* Open files in 'rb' mode to avoid translation on Windows

Previously tests simply relied on files that were present in the
repository to run tests using open().  Change the filenames to ensure
that no longer happens.

requests_mock doesn't have a way to match against the request body for
PUT/POST; an attempt to add a new Matcher to do that worked but it
needs to subclass the currently private adapter._Matcher class or
duplicate most of its functionality.

Change-Id: I8c30b41db20af8ecafe67e760e872fc08adec905
Merge "Update use of open() in object API"