Fixing the issue #123

Here is what the fix does:
1. If the parameter has inline constraint but is optional as well, it creates OptionalRouteConstraint -- which is new type of constraints added -- for that parameter. The OptionalRouteConstraint takes the original IRouteConstraint that would have been created for this parameter as input.

2. The 'Match' method on OptionalRouteConstraint checks if the value for the parameter is present. If it is it delegates to the 'Match' method of the original IRouteConstraint. If the value is not present, it returns true. 
Don't use `DoesNotThrowAsync` -- they removed it from newer versions of xUnit. Also you have no reason to guard against throwing... a throwing test is a failing test.