LIBCLOUD-433: Make storage backend send content-type of
"application/octet-stream" if none is supplied and none can be guessed.

Also update affected tests.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Make 'DEFAULT_CONTENT_TYPE' a constant and add a test case for it.
Update CHANGES.
Add "strict_mode" to the storage driver.

When this mode is enabled (disabled by default), it will default to an old
behavior when no content type is provided and none can't be automatically
detected.
docs: Update upgrade notes.
Use unittest2.