BUG Text::BigSummary() fails with undefined $data when $plain = false
Update cache-control.md

Added useful information after half an hour trying to have cache working :)
FIX: Prevent JSON response showing when re-opening closed tab (fixes silverstripe/silverstripe-cms#1121)
BUG File attach handler is no longer accessible if attachment is disallowed or disabled
fixed and tested object has_extension
Merge pull request #3575 from kinglozzer/pulls/fix-reopen-tab-response

FIX: Prevent JSON response showing when re-opening closed tab (fixes silverstripe/silverstripe-cms#1121)
Merge pull request #3484 from dnadesign/object_has_extension_fix

fixed and tested object has_extension
Session keys are case sensitive
Merge pull request #3578 from g4b0/patch-4

Session keys are case sensitive
Merge pull request #3576 from tractorcow/pulls/3.1/upload-attach

BUG File attach handler is no longer accessible if attachment is disallowed or disabled
Merge pull request #3521 from halkyon/bigsummary_plain_fix

BUG Text::BigSummary() fails with undefined $data when $plain = false