Add appropriate HTTP headers to repository requests (#1486)

Dalamud/Plugin/Internal/Types/PluginRepository.cs

@@ -37,10 +37,18 @@ internal class PluginRepository
         Timeout = TimeSpan.FromSeconds(20),
         DefaultRequestHeaders =
         {
+            Accept =
+            {
+                new MediaTypeWithQualityHeaderValue("application/json"),
+            },
             CacheControl = new CacheControlHeaderValue
             {
                 NoCache = true,
             },
+            UserAgent =
+            {
+                new ProductInfoHeaderValue("Dalamud", Util.AssemblyVersion),
+            },
         },
     };