Use return value of last filter when multiple filters are registered.

problem 1.
  filter's return value null is ignored if one of following filters returns non-null value;

problem 2.
  if filter returns new object instead of 'jsonLog' argument and following filter exists,
  the returned value is overwritten by value from following filters.
Merge pull request #18 from zaki50/fix_multiple_filters_issue

Use return value of last filter when multiple filters are registered.