Truncate local file in ScpClient.Download

Similar to sshnet#1686 but for the local side of SCP: opening the file should use
Create not OpenWrite.

closes sshnet#648

Author: Rob-Hague

src/Renci.SshNet/ScpClient.cs

@@ -831,7 +831,7 @@ private void InternalDownload(IChannelSession channel, Stream input, FileSystemI
                         fileInfo = new FileInfo(Path.Combine(currentDirectoryFullName, fileName));
                     }
 
-                    using (var output = fileInfo.OpenWrite())
+                    using (var output = fileInfo.Open(FileMode.Create, FileAccess.Write))
                     {
                         InternalDownload(channel, input, output, fileName, length);
                     }

test/Renci.SshNet.IntegrationTests/ScpTests.cs

@@ -679,7 +679,8 @@ public void Scp_Download_FileInfo_ExistingFile(IRemotePathTransformation remoteP
                 }
             }
 
-            var fileInfo = new FileInfo(Path.GetTempFileName());
+            // Create a local file larger than the remote file in order to test truncation.
+            var fileInfo = new FileInfo(CreateTempFile(size + 64));
 
             try
             {