Align AspNet versions; improve default experience

src/BaGet.AWS/BaGet.AWS.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;net461</TargetFrameworks>
@@ -8,7 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="AWSSDK.S3" Version="3.3.30" />
-    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="2.1.1" />
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="2.2.0" />
   </ItemGroup>
 
   <ItemGroup>

src/BaGet.Core/Configuration/FileSystemStorageOptions.cs

@@ -1,14 +1,28 @@
-using System.IO;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.IO;
+using System.Linq;
 
 namespace BaGet.Core.Configuration
 {
-    public class FileSystemStorageOptions : StorageOptions
+    public class FileSystemStorageOptions : StorageOptions, IValidatableObject
     {
         /// <summary>
         /// The path at which content will be stored. Defaults to the same path
         /// as the main BaGet executable. This path will be created if it does not
         /// exist at startup. Packages will be stored in a subfolder named "packages".
         /// </summary>
-        public string Path { get; set; } = Directory.GetCurrentDirectory();
+        public string Path { get; set; }
+
+        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+        {
+            // Convert an empty storage path to the current working directory.
+            if (string.IsNullOrEmpty(Path))
+            {
+                Path = Directory.GetCurrentDirectory();
+            }
+
+            return Enumerable.Empty<ValidationResult>();
+        }
     }
 }