attribute 'isRequired': 'True' is not a valid value of the atomic type 'xs:boolean'

[pwhamman]

Original issue #136

In v2.2.0 I'm receiving this error when specifying the isRequired the issue was fixed in v2.1.9 but somehow seems that it got lost.

attribute 'isRequired': 'True' is not a valid value of the atomic type 'xs:boolean'

[pitbulk]

It seems I accidentally reverted #136 change, applied it again.