Better hash code
Better hash code

Standard hash code impl: hash = hash * 31 + value.hashCode
Joda-Beans incorrectly had: hash += hash * 31 + value.hashCode
This caused poor hash codes when most properties of a bean were null
Fixes #89

JodaOrg/joda-beans,fengshao0907/joda-beans,JodaOrg/joda-beans,fengshao0907/joda-beans