Update PropertiesInvocationHandler.java

We try to find the unexpanded key in propertiesManager to locate default value when the expandedKey is not found.
Update KeyExpansionTest.java

We want to obtain default value if anything goes wrong in locating expanded key in our source.
Merge pull request #84 from andreipet/master

Use of default value for for properties using the Key Expansion mechanism.
To be revised.