Harden HMAC verification. props duck_. [28053] for 3.7.

Built from https://develop.svn.wordpress.org/branches/3.7@28055


git-svn-id: http://core.svn.wordpress.org/branches/3.7@27885 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Bump Akismet external in the 3.7 branch to 2.6.0.


git-svn-id: http://core.svn.wordpress.org/branches/3.7@27886 1a063a9b-81f0-0310-95a4-ce76da25c4cd
3.7.2

Built from https://develop.svn.wordpress.org/branches/3.7@28056


git-svn-id: http://core.svn.wordpress.org/branches/3.7@27888 1a063a9b-81f0-0310-95a4-ce76da25c4cd