The PR resolves an issue reported under #133.

While testing against https://hub.docker.com/_/registry, "Docker-Content-Digest" headers were found on response.Headers collection rather than response.Content.Headers.
Aligned internal/private variables to use _camelCase as there was some inconsistencies.

Signed-off-by: Daniel Robinson <daniel.robinson@pebble.tv>

src/OrasProject.Oras/Content/Digest.cs

@@ -20,16 +20,16 @@ namespace OrasProject.Oras.Content;
 
 internal static class Digest
 {
-    private const string digestRegexPattern = @"[a-z0-9]+(?:[.+_-][a-z0-9]+)*:[a-zA-Z0-9=_-]+";
-    private static readonly Regex digestRegex = new Regex(digestRegexPattern, RegexOptions.Compiled);
+    private const string _digestRegexPattern = @"[a-z0-9]+(?:[.+_-][a-z0-9]+)*:[a-zA-Z0-9=_-]+";
+    private static readonly Regex _digestRegex = new Regex(_digestRegexPattern, RegexOptions.Compiled);
 
     /// <summary>
     /// Verifies the digest header and throws an exception if it is invalid.
     /// </summary>
     /// <param name="digest"></param>
     internal static string Validate(string? digest)
     {
-        if (string.IsNullOrEmpty(digest) || !digestRegex.IsMatch(digest))
+        if (string.IsNullOrEmpty(digest) || !_digestRegex.IsMatch(digest))
         {
             throw new InvalidDigestException($"Invalid digest: {digest}");
         }

src/OrasProject.Oras/Registry/Remote/HttpResponseMessageExtensions.cs

@@ -25,6 +25,8 @@ namespace OrasProject.Oras.Registry.Remote;
 
 internal static class HttpResponseMessageExtensions
 {
+    private const string _dockerContentDigestHeader = "Docker-Content-Digest";
+
     /// <summary>
     /// Parses the error returned by the remote registry.
     /// </summary>
@@ -75,7 +77,7 @@ public static async Task<Exception> ParseErrorResponseAsync(this HttpResponseMes
     /// <exception cref="NotImplementedException"></exception>
     public static void VerifyContentDigest(this HttpResponseMessage response, string expected)
     {
-        if (!response.Content.Headers.TryGetValues("Docker-Content-Digest", out var digestValues))
+        if (!response.Headers.TryGetValues(_dockerContentDigestHeader, out var digestValues))
         {
             return;
         }
@@ -154,7 +156,7 @@ public static async Task<Descriptor> GenerateDescriptorAsync(this HttpResponseMe
 
         // 4. Validate Server Digest (if present)
         string? serverDigest = null;
-        if (response.Content.Headers.TryGetValues("Docker-Content-Digest", out var serverHeaderDigest))
+        if (response.Headers.TryGetValues(_dockerContentDigestHeader, out var serverHeaderDigest))
         {
             serverDigest = serverHeaderDigest.FirstOrDefault();
             if (!string.IsNullOrEmpty(serverDigest))