Fix PKCS#7 parsing on macOS
Fix PKCS#7 parsing on macOS

On macOS, X509Certificate2Collection.Import() throws the following exception when importing PKCS#7 data:
> HResult = -25257  
Message = Unknown format in import.  
Source = System.Security.Cryptography.X509Certificates  
StackTrace = at Internal.Cryptography.Pal.X509Pal.AppleX509Pal.GetCertContentType(Byte[] rawData)  
   at System.Security.Cryptography.X509Certificates.X509Certificate2.GetCertContentType(Byte[] rawData)  
   at Internal.Cryptography.Pal.StorePal.FromBlob(Byte[] rawData, SafePasswordHandle password, X509KeyStorageFlags keyStorageFlags)  
   at System.Security.Cryptography.X509Certificates.X509Certificate2Collection.Import(Byte[] rawData, String password, X509KeyStorageFlags keyStorageFlags)  
   at System.Security.Cryptography.X509Certificates.X509Certificate2Collection.Import(Byte[] rawData)  
   at PeNet.Header.Authenticode.AuthenticodeInfo.GetSigningCertificateNonWindows(Byte[] pkcs7)
   
Using `SignedCms.Decode` instead of `X509Certificate2Collection.Import` as [suggested by Jeremy Barton][1] works on macOS thus making all the unit tests pass.

[1]: https://github.com/dotnet/runtime/issues/15073#issuecomment-374787612
secana/PeNet