CVE-2013-0156: Safe XML params parsing. Doesn't allow symbols or yaml.
bumping to 2.3.15 :cry::gun:
Revert "bump up rack version to the one that includes the Hash DoS fix"

Rack 1.1.3 also changes the Set-Cookie header to expects a
newline-delimited string instead of an Array, which breaks Rails 2.3's
expectations in a variety of ways.

This reverts commit 27a508c91e04b619c8a1eb50ef115b226b21182c.

Conflicts:
	actionpack/Rakefile
Fix for CVE-2013-0155
Docs: Fixed bad exists? documentation.

Base#exists? does not actually take options like finder methods. Trying
to use what the documentation suggests will return a PG error because it
will look for a column named 'conditions'.

I changed the documentation to reflect how the exists? method actually
works.