Improve code coverage

[keilw]

The code coverage is currently 40%, see codecov badge. We should create more JUnit tests especially in packages with a very low coverage. At least 60% like the JSR 363 RI should be archived, if more, even better.

[filipvanlaenen]

The codecov badge says 50% today! :-)

[teobais]

Currently at 59%, but it should definitely improve. Maybe a good candidate for our OC1 Hackergarten.

[keilw]

Yes, it is not a killer criteria but certainly a low hanging fruit. Could be combined e.g. with increasing the use of Hamcrest where appropriate.