Fix for lost HttpHeader items

In cases where there are multiple header entries with the same name,
only the first one in the collection was being copied from the OkHttp
Header to the HttpResponseMessage.Headers collection

Author: dwhathaway

src/ModernHttpClient/Android/OkHttpNetworkHandler.cs

@@ -152,8 +152,10 @@ protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage
 
             var respHeaders = resp.Headers();
             foreach (var k in respHeaders.Names()) {
-                ret.Headers.TryAddWithoutValidation(k, respHeaders.Get(k));
-                ret.Content.Headers.TryAddWithoutValidation(k, respHeaders.Get(k));
+				foreach (string item in respHeaders.Values(k)) {
+					ret.Headers.TryAddWithoutValidation(k, item);
+					ret.Content.Headers.TryAddWithoutValidation(k, item);
+				}               
             }
 
             return ret;