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. 
nit: I tend to do `Assert.Single(compositeConstraint.Constraints, c => is MinLengthConstraint)` for this kind of thing. It's up to you if you think it's better or not.

Basically if something looks verbose in xUnit, odds are they provided a convenient way to do it.