Ensure _chooseSplitIndex always return a number

[DiogoDoreto]

Fix #69

Current implementation of _chooseSplitIndex may return undefined when all children have infinity area.

When all inserted rects are infinity, they will be all inserted in a single node, instead of properly balancing the tree (updated test to validate this).

When data is random, an incorrect split may occur in a non-leaf node, causing a non-leaf node to have 0 children and causing error #69 when the tree will try to insert new data in its path.

This fix ensure that _chooseSplitIndex always return a number.

[mourner]

This is fantastic, thank you so much for finding a great fix!

[borodaev]

Great!