Fix RSACryptoServiceProvider::LegalKeySize on Unix

The Unix RSACryptoServiceProvider type didn't set _legalKeySizesValue, or override the LegalKeySizesValue, and so null was being returned.

Author: bartonjs

src/System.Security.Cryptography.RSA/src/System/Security/Cryptography/RSACryptoServiceProvider.Unix.cs

@@ -17,11 +17,13 @@ public sealed partial class RSACryptoServiceProvider : RSA, ICspAsymmetricAlgori
         public RSACryptoServiceProvider()
         {
             _defer = new RSAOpenSsl();
+            _legalKeySizesValue = _defer.LegalKeySizes;
         }
 
         public RSACryptoServiceProvider(int dwKeySize)
         {
             _defer = new RSAOpenSsl(dwKeySize);
+            _legalKeySizesValue = _defer.LegalKeySizes;
         }
 
         public RSACryptoServiceProvider(int dwKeySize, CspParameters parameters)

src/System.Security.Cryptography.RSA/tests/KeyGeneration.cs

@@ -8,14 +8,12 @@ namespace System.Security.Cryptography.Rsa.Tests
     public class KeyGeneration
     {
         [Fact]
-        [ActiveIssue(1984, PlatformID.AnyUnix)]
         public static void GenerateMinKey()
         {
             GenerateKey(rsa => GetMin(rsa.LegalKeySizes));
         }
 
         [Fact]
-        [ActiveIssue(1984, PlatformID.AnyUnix)]
         public static void GenerateSecondMinKey()
         {
             GenerateKey(rsa => GetSecondMin(rsa.LegalKeySizes));