#1283 Fall Through MDL Bond Type of MDLV2000Writer is 0

[John May]

The default bond type when writing MDL defaults to '0' which is an invalid bond type and won't be read elsewhere. Not sure what should be the default, perhaps 'Any' (8) or maybe it should through an exception?

https://github.com/egonw/cdk/blob/master/src/main/org/openscience/cdk/io/MDLV2000Writer.java#L403