XCOMMONS-636: Observation manager does not fully overwrite existing event listener

(cherry picked from commit 1c148320c37bcd1353af737f17b0e1632fdb502c)

Conflicts:
	xwiki-commons-core/xwiki-commons-observation/xwiki-commons-observation-local/src/test/java/org/xwiki/observation/ObservationManagerTest.java

(cherry picked from commit 8ff948cfc98a3bf48e75df9af7d1c32b1600e379)
(cherry picked from commit 3cd579472ab43c9f5b2f09f60e9ddfa080e7244d)