readStream: certain values of start/end cause an error

[chesles]

Specifically 0 causes this error. Not sure what other values do.

var lu = require('levelup')
lu('test', {createIfMissing:true}, function(err, db) {
    var s = db.readStream({start: 0}).on('data', console.log);
});

node crashes with this error:

terminate called throwing an exceptionAbort trap: 6

Using {start: '0'} gives expected results.

[rvagg]

Thanks @chesles, the fix is in 5dcbc5b, caused by checking falsey value rather than a string != undefined.
Pushed to npm as @0.1.2