Bug 28489: Modify sessions.a_session from longtext to longblob

Bug 28489: Modify sessions.a_session from longtext to longblob

This column contains serialized data, it should be a blob.
And it fixes encoding issues.

Test plan:
1. Apply patch and run updatedatabase
2. In $KOHA_CONF make sure that you have
   <strict_sql_modes>0</strict_sql_modes>
3. Create 2 libraries:
  a. TESTÄ/Testä
  b. TEST✓/Test✓
4. Go to the staff interface, change your library to Testä, then
   navigate to a few other pages, make sure there is no encoding problem
   with the library name in the top right corner
5. Do the same with Test✓

Signed-off-by: Petro Vashchuk <stalkernoid@gmail.com>
Signed-off-by: Andrew Nugged <nugged@gmail.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>