Fixed issue with Owin delegate

- Changed OwinStartupBase to use OwinHelpers extension method

I couldn't get the samples for owin to work, so i tried to change UseSimpleWeb to be compatible with "Owin" signature.

I could not get rake full to run, something with:
error: Could not find file: C:\src\fromgit\Simple.Web\src\Simple.Web.Razor.Tests.ExternalDummyAssembly\bin\Release\xunit
.dll

So seems like it's trying to run tests in ExternalDummyAssembly and failing on that because that assembly doesn't have a reference to xunit. I'm not that keen on rake/ruby but guees it should not try to run tests in ExternalDummyAssembly.

Please advice