Remove redundant null check in Pair.cs.

'is' will return false if it is null in this case (as it isn't a Pair in that case)
Fix ToString formatting for BezierCurve and Seg2F.

Seg2F just didn't need th extra params, while BezierCurve was missing three parameters.
Fix an exception message within a DomNodePropertyMatch constructor.

ArgumentNullExceptions are constructed with the parameter that is null being specified first as a string, then the message follows.
Merge pull request #2 from lioncash/minor-cleanup

Removed redundant null check in Pair.cs.
Fixed ToString() formatting for BezierCurve and Seg2F.
Fixed an exception message within a DomNodePropertyMatch constructor.
(Thanks to lioncash for finding and fixing these.)