String minlength and maxlength don't work

The **String** type needs `minlength` and `maxlength` options to be in lowercase as specified in the **mongoose** docs:

[SchemaStringOptions.prototype.maxlength](https://mongoosejs.com/docs/api.html#schemastringoptions_SchemaStringOptions-maxlength)
[SchemaStringOptions.prototype.minlength
](https://mongoosejs.com/docs/api.html#schemastringoptions_SchemaStringOptions-minlength)
PR #20 