Fix ReSharper's "Use string interpolation expression" and "Replace if statement with null-propagating code" / Code Smell (#240)

Author: marodev

dropbox-sdk-dotnet/Dropbox.Api/DropboxAppClient.cs

@@ -62,7 +62,7 @@ private static string GetBasicAuthHeader(string appKey, string appSecret)
                 throw new ArgumentNullException("appSecret");
             }
 
-            var rawValue = string.Format("{0}:{1}", appKey, appSecret);
+            var rawValue = $"{appKey}:{appSecret}";
             return Convert.ToBase64String(Encoding.UTF8.GetBytes(rawValue));
         }
     }

dropbox-sdk-dotnet/Dropbox.Api/DropboxRequestHandler.cs

@@ -430,10 +430,7 @@ private async Task<Result> RequestJsonStringWithRetry(
             }
             finally
             {
-                if (body != null)
-                {
-                    body.Dispose();
-                }
+                body?.Dispose();
             }
         }