Make UrlHelper methods virtual

[rynowak]

@kulmugdha moved from aspnet/Routing#80

On the routing blog http://blogs.msdn.com/b/webdev/archive/2014/06/20/asp-net-vnext-routing-overview.aspx, a user expressed that he wants to slightly modify the generated URL on its way out. If we don't make the methods virtual, he has to do either of following.

1. implement IUrlHelper from scratch
2. Implement IUrlHelper and wrap an instance of UrlHelper in the implementation.
3. Extend UrlHelper and use Explicit Interface Implementation

None of these is very optimal.

[kichalla]

Just FYI...one more scenario that I have come across: Generate a link to a local or cdn server url. In this case the user is currently having an extension on UrlHelper...making the Content method virtual would help in this case...

[kichalla]

Resolved as part of commit: 43c7ddb