#348 Mapping parameters based on name when other mappings fail

After introducing nested properties (#65) also the option was created to map from a parameter directly (a non bean mapping). This PR introduces mapping based on on name when all other options fail of parameters.

I initially added a check if there are unused source parameters (and raise a warning when this is the case). However this does not work very well with expressions. In expressions you can refer to parameters without these parameters being marked as 'used'. Expression tests start to fail when this error handling was introduced.

Some cleanup needs to be added to the next release, were superflous null checks are removed. I'll write a PR for that.