Tests for CollectingAndReducing.
Merge branch 'master' into solutions

Conflicts:
	src/main/java/de/codecentric/java8examples/streaming/CollectingAndReducing.java
	src/test/java/de/codecentric/java8examples/streaming/CollectingAndReducingTest.java
Fix one test.
Solutions
Merge branch 'master' into solutions
One more (crazy) solution.
Revivive accidentally deleted method Invoice.getTotal.
JavaDoc for default method implementation
Add an example showing how to work with unrealted default methods
Add example for using a method reference to implemente a function
Merge branch 'workshop-changes'