Looks like you're having a similar problem as me: Binary data corrupted when hosting ServiceStack in Mono + FastCGI - f7e is probably the data size (3966 bytes).
Edit:
This is due to chunked transfers (introduced in HTTP 1.1). I was able to work around my problem by forcing the request to use HTTP 1.0.
On my question knocte suggested a patch to mono that might fix your problem:
Patch: https://bugzilla.xamarin.com/attachment.cgi?id=3356
File: https://github.com/mono/mono/blob/master/mcs/class/System.Web/System.Web/HttpResponse.cs