Only update registration properties for which a non-null value has been
provided
Move Clietn update code from registry impl to UpdateClient in order to
better encapsulate the update strategy (this makes it easier to provide
an alternatve implementation of ClientRegistry),
Merge remote-tracking branch 'origin/master' into
Registration-Update-Fix

Conflicts:
	leshan-core/src/main/java/leshan/server/lwm2m/client/Client.java
	leshan-core/src/main/java/leshan/server/lwm2m/client/ClientRegistryImpl.java
Added JavaDoc, make "port" property an Integer in order to better allow
for null value
nit-picking :)
Merge branch 'sophokles73-Registration-Update-Fix'