fix(CSS): Do not use CSS position fixed for thead on IE <= 9 (refs #4824)
Merge pull request #1192 from eguaj/3.2-A4824

fix(CSS): Do not use CSS position fixed for thead on IE <= 9 (closes #4824)
fix(Session): Revert to non-blocking sessions (refs #4825)

Commit 3e47c82708155949b5059909cebd375eb53f19f0 introduced a regression
that blocked concurrent requests with same session id.

This reverts to the original non-blocking behaviour.