Ensure that setting wxChoice height to its default value does set it.

Ensure that setting wxChoice height to its default value does set it.

Setting wxChoice height to its default value didn't change the height even if
the current height was different from the default. This resulted in problems
when a wxChoice was used inside a wxAuiToolBar because AUI temporarily reduces
the toolbar size to (1, 1) when docking it (thus ensuring that the height of
wxChoice is changed too) and generally didn't make sense.

Fix this by resetting the height to the default value if the value passed to
wxChoice::SetSize() is what it considers to be its default height.

Add a unit test for this bug and also add a wxChoice to AUI sample to allow
testing for wxChoice behaviour inside a wxAuiToolBar being [un]docked.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63708 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775