Steps to reproduce the issue

Install plgSpambotCheck_j3.0.0_1.3.12.zip through extension manager an Joomla! 3.4.x installation (which you can download from our Joomla! extensions download menu http://www.vi-solutions.de/en/downloads-spambotcheck/joomla-extension-plugin-spambotcheck-1-3-12)
Go to Menu "Update" in extension manager
Click "Find Updates", you will see an update for the Plugin in the list then
Check the update and click "Update"

Expected result

Display of custom installation message (see )

Actual result

Message is missing. See

System information (as much as possible)

Tested on
Joomla! 3.4.0 or 3.4.1
php 5.4.7, 5.4.21 on XAMPP and php 5.3.29 on productive Linux server

Additional comments

The userState, that contains the message, is cleared in the model administrator/components/com_installer/models/update.php function populateState
$app->setUserState('com_installer.message', '');
$app->setUserState('com_installer.extension_message', '');
As this function is called more than once during the update on the second call the message ist always ''.
Actually I have never seen a Joomla! extension before, that clears the userState in the model and I think this is the wrong place.