Re-add virtual to intended overrideable methods

While there are lots of places in the code where `virtual` is there simply because
the Java auto-port treated an absence of `final` as intending overrideability, these
two places are INTENDED override points.

Author: sblom

src/json-ld.net/Core/DocumentLoader.cs

@@ -45,13 +45,13 @@ JsonLDContentType GetJsonLDContentType(string contentTypeStr)
         }
 
         /// <exception cref="JsonLDNet.Core.JsonLdError"></exception>
-        public RemoteDocument LoadDocument(string url)
+        public virtual RemoteDocument LoadDocument(string url)
         {
             return LoadDocumentAsync(url).GetAwaiter().GetResult();
         }
 
         /// <exception cref="JsonLDNet.Core.JsonLdError"></exception>
-        public async Task<RemoteDocument> LoadDocumentAsync(string url)
+        public virtual async Task<RemoteDocument> LoadDocumentAsync(string url)
         {
             RemoteDocument doc = new RemoteDocument(url, null);
             try