cdn 6.x-2.6

Changes since 6.x-2.5:

• #1893170 by c4doug | martinjbaker: Fixed Patch file fails with Drupal 6.28.
• #1557928 by Wim Leers: Don't require CSS aggregation to be enabled when the AdvAgg module is enabled
• #1634932 by Wim Leers: CSS aggregation with HTTPS + clear cache: CDN's CSS cache not actually cleared
• Fixed #1576458: PHP Error: Function Redeclare
• #1572794: Move constant declarations to their own file
• Fixed #1552456: CDN module says there's a problem with the configuration, while it may be deliberate
• Fixed #1593930 by mstrelan and Wim Leers: cdn_get_domains() is broken for auto-balancing when using protocol-relative domains: fix + tighten tests
• Fixed #1623252: Only rewrite image links when they have the same file extension as the image's
• Problem with cdn_admin_other_settings_form_validate() and cdn_pick_server()
• After install: erroneous call to variable_get()
• #1549582: Problem with CDN CSS aggregation with multiple stylesheets with same filename