Revert "Fix for issue #156"

This reverts commit 8decf32f8fd261fe88f43991048627df4cb727f7.
Revert "correct fix for #156", to be replaced with 400-error return.

This reverts commit 7ac858cecff35f27d40a561993192f20c7e221ed.
Added Choice parsing for DateTime
return 400 Bad Request when invalid if-modified-since header is received