[BUGFIx] Checks ext key length with underscores

Due to the table field length of 30 chars in
TER table extension_keys the check for the length
of the extension key must be done without removing
all underscores.

Change-Id: I37d2c3934111d9acc7c68574a86a6e977a1d0f42
Reviewed-on: http://review.typo3.org/31727
Reviewed-by: Christian Zenker <typo3@xopn.de>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>