Check for empty user agent to avoid notice.

Props sergej.mueller
fixes #22979


git-svn-id: http://core.svn.wordpress.org/trunk@24275 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Don't show both "update failed" and "update successful" messages when upgrading a theme fails due to connectivity problems.

Props SergeyBiryukov
fixes #23027


git-svn-id: http://core.svn.wordpress.org/trunk@24276 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Fix 'Collapse' button in RTL. props SergeyBiryukov. see [22977]. fixes #23846.

git-svn-id: http://core.svn.wordpress.org/trunk@24277 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: updated screenshot, smaller (82 to 55 KB) and a bit less cow.

git-svn-id: http://core.svn.wordpress.org/trunk@24278 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: update Quote styles so that fancy display only affects the actual quote content. Fixes #24332.

git-svn-id: http://core.svn.wordpress.org/trunk@24279 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-beta3-24280.

git-svn-id: http://core.svn.wordpress.org/trunk@24280 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: fixes to remove invalid CSS rules, including unneeded  IE-specific filters and `calc()` values.

git-svn-id: http://core.svn.wordpress.org/trunk@24281 1a063a9b-81f0-0310-95a4-ce76da25c4cd