When processing bandwidth attributes per session
level, always update them, independently if the
currently parsed value has been set => 0.
The problem is, that if we only set them,
when initialized to != -1, then every new offer
within existing dialog, will keep bandwidth set
for the monologue, even if the newer offer
doesn't set this anymore.
Change-Id: Ifef964798a8ce9b38a4fd5f26b5bb4d0678b8829