RFR: 8242001: ChoiceBox: must update value on setting SelectionModel, part2

[Jeanette Winzenburg]

The issue is that ChoiceBox didn't sync its value to the replaced SelectionModel's selectedItem if the item is null.

Fixed by removing the constraint from selectionModel property.

Added tests that failed before and passed after the fix. Note that I also changed a test method in ChoiceBoxTest with
an incorrect test assumption (which passed for the incorrect implementation).

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Commit messages:
 - 8242001: ChoiceBox: must update value when setting a SelectionModel,

Changes: https://git.openjdk.java.net/jfx/pull/207/files
 Webrev: https://webrevs.openjdk.java.net/jfx/207/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242001
  Stats: 46 lines in 3 files changed: 42 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/207.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/207/head:pull/207

PR: https://git.openjdk.java.net/jfx/pull/207