Fixes exception when DnnImageHandler returns a cached response

[daguiler]

Fixes #3872

Summary

This error occurs when the image was already generated by DnnImageHandler in a previous request and now it's cached in the ~\App_Data\_imagecache folder.

A quick workaround is to switch server cache off, in the web config:

<configuration>
  ...
  <appSettings>
    <add key="DnnImageHandler" value="enableclientcache=False;enableservercache=False" />
    ...

but then of course all images are re-generated everytime.

As explained here and here, the problem is that the System.Web.HttpResponseWrapper.TransmitFile(string) method, followed by Response.End() produces the exception mentioned in #3872.

This sequence can be seen in code, in following lines:

Dnn.Platform/DNN Platform/Library/Services/GeneratedImage/DiskImageStore.cs

Line 291 in 1bb11bf

response.TransmitFile(path);

Dnn.Platform/DNN Platform/Library/Services/GeneratedImage/ImageHandlerInternal.cs

Line 257 in 1bb11bf

context.Response.End();

Replacing Response.End() with Response.Flush() fixed the issue.

[david-poindexter]

Looks good to me - thanks @daguiler