Allow to use wait and grow with 0 as integer

This patch fix a problem with {wait: 0, grow: 0}, it was changing the values to the default ones. This fix prevent this behavior.

*For a temporary fix, set the values as string: {wait: '0', grow: '0'}
Merge pull request #45 from danielsantiago/patch-2

Allow to use wait and grow with 0 as integer