Fix UAA startup if postgresql is used for session store

[strehle]

found by accident on postgresql startup
Same as #1769 but then we cannot use database session store for postgresql. Without base64, it works, see
spring-projects/spring-session#1201 Because null value would be written into postgresql with error, see https://stackoverflow.com/questions/53160867/postgres-error-invalid-byte-sequence-for-encoding-utf8-0xca-0x2d

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/185676060

The labels on this github issue will be updated when the story is started.

[strehle]

if postgresql is used for session store set in uaa.yml
servlet.session-cookie.encode-base64 : false
e.g.

servlet:
  session-cookie:
    encode-base64: false

[hsinn0]

So both this and the related uaa-release change keep the default as base64 encoding. That is good enough then.