fix test spellling
add StringObservable NPE tests
Merge branch 'master' of https://github.com/davidmoten/rtree
Merge pull request #4 from maxamel/master

Add unit tests.
Thanks Max
Merge branch 'master' of https://github.com/davidmoten/rtree
remove unused imports and format
Merge pull request #3 from davidmoten/master

Merge DaveMoten changes for 26/9