Fix #83: Support for boolean sortObjectKeys prop

[nhunzaker]

Documentation suggests that passing a boolean to the sortObjectKeys prop should provide a default sorting method. This commit adds a type check to support both boolean and comparator functions.

Otherwise, invoking array.sort(true) yields:

TypeError: invalid Array.prototype.sort argument[

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Fixes #83

[zalmoxisus]

Thanks for the contribution! I changed it to be a bit shorter. I think throwing when the argument is not boolean and not function is ok, helping debugging.