Add netstandard2.0 support (#33)

* Add netstandard2.0 support

src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs

@@ -0,0 +1,26 @@
+namespace System.Net.Http;
+
+internal static class HttpClientExtensions
+{
+    public static async Task<Stream> ReadAsStreamAsync(this HttpContent content, CancellationToken cancellationToken)
+    {
+        if (content is null)
+        {
+            throw new ArgumentNullException(nameof(content));
+        }
+
+        cancellationToken.ThrowIfCancellationRequested();
+        return await content.ReadAsStreamAsync();
+    }
+
+    public static async Task<string> ReadAsStringAsync(this HttpContent content, CancellationToken cancellationToken)
+    {
+        if (content is null)
+        {
+            throw new ArgumentNullException(nameof(content));
+        }
+
+        cancellationToken.ThrowIfCancellationRequested();
+        return await content.ReadAsStringAsync();
+    }
+}