More samples around setting content types.

Please check my assertions regarding:
* [updating an existing object's content type](https://github.com/ycombinator/php-opencloud/compare/rackspace:working...ycombinator:more-object-store-samples?expand=1#diff-eb570f7d1f364f397407d304e7c0a5aaR53), and
* [when to specify a content type when uploading a new object](https://github.com/ycombinator/php-opencloud/compare/rackspace:working...ycombinator:more-object-store-samples?expand=1#diff-4789015b5e21ffac104ac9d685920f87R44).

Attn: @gecampbell, @jamiehannaford