Support ASP.NET vNext

This is the first part of #170

/cc @lodejard @davidfowl @Eilon @glennc
This is causing an exception.

    System.InvalidOperationException: Failed to compare two elements in the array. ---> System.Security.Cryptography.CryptographicException: Hash not valid for use in specified state.
      at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr)
      at System.Security.Cryptography.Utils.HashData(SafeHashHandle hHash, Byte[] data, Int32 cbData, Int32 ibStart, Int32 cbSize)
      at System.Security.Cryptography.Utils.HashData(SafeHashHandle hHash, Byte[] data, Int32 ibStart, Int32 cbSize)
      at System.Security.Cryptography.SHA1CryptoServiceProvider.HashCore(Byte[] rgb, Int32 ibStart, Int32 cbSize)
      at System.Security.Cryptography.HashAlgorithm.ComputeHash(Stream inputStream)
      at Xunit.Sdk.TestMethodTestCase.GetUniqueID()