FastCgi: ignore SocketException when closing the connection

By using --trace=E:all I've detected that this exception could happen at
the end of a connection under high load, in which case the EndRequest()
method would not complete, and some buffers would not be released.

For the record, the full stacktrace was:

[0x7ff54ceff700:] EXCEPTION handling: System.Net.Sockets.SocketException: The socket is not connected

"Threadpool worker" tid=0x0x7ff54ceff700 this=0x0x7ff54eab4930 thread handle 0x483 state : not waiting owns ()
  at System.Net.Sockets.Socket.Shutdown (System.Net.Sockets.SocketShutdown) [0x00051] in /home/andres1304/Code/system_mono/mono/mcs/class/System/System.Net.Sockets/Socket_2_1.cs:1416
  at Mono.FastCgi.StandardSocket.Close () [0x00002] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/StandardSocket.cs:68
  at Mono.FastCgi.Connection.EndRequest (uint16,int,Mono.FastCgi.ProtocolStatus) [0x000cc] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Connection.cs:342
  at Mono.FastCgi.Request.CompleteRequest (int,Mono.FastCgi.ProtocolStatus) [0x0003a] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Request.cs:82
  at Mono.WebServer.FastCgi.Responder.CompleteRequest (int) [0x00001] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Responder.cs:135
  at (wrapper remoting-invoke-with-check) Mono.WebServer.FastCgi.Responder.CompleteRequest (int) <IL 0x00039, 0xffffffff>
  at (wrapper xdomain-dispatch) Mono.WebServer.FastCgi.Responder.CompleteRequest (object,byte[]&,byte[]&,int) <IL 0x0005c, 0xffffffff>
  at (wrapper xdomain-invoke) Mono.WebServer.FastCgi.Responder.CompleteRequest (int) <0xffffffff>
  at (wrapper remoting-invoke-with-check) Mono.WebServer.FastCgi.Responder.CompleteRequest (int) <IL 0x00039, 0xffffffff>
  at Mono.WebServer.FastCgi.WorkerRequest.CloseConnection () <0x0003b>
  at Mono.WebServer.FastCgi.WorkerRequest.FlushResponse (bool) <0x00024>
  at System.Web.HttpResponseStream.Flush (System.Web.HttpWorkerRequest,bool) <0x000ba>
  at (wrapper remoting-invoke-with-check) System.Web.HttpResponseStream.Flush (System.Web.HttpWorkerRequest,bool) <0xffffffff>
  at System.Web.HttpResponse.Flush (bool) <0x001cb>
  at System.Web.HttpApplication.OutputPage () <0x00077>
  at System.Web.HttpApplication.PipelineDone () <0x000eb>
  at System.Web.HttpApplication/<Pipeline>c__Iterator3.MoveNext () <0x05003>
  at System.Web.HttpApplication.Tick () <0x00051>
  at System.Web.HttpApplication.Start (object) <0x001a7>
  at System.Web.HttpApplication.System.Web.IHttpHandler.ProcessRequest (System.Web.HttpContext) <0x00043>
  at System.Web.HttpRuntime.Process (System.Web.HttpWorkerRequest) <0x0022f>
  at System.Web.HttpRuntime.RealProcessRequest (object) <0x0009f>
  at System.Web.HttpRuntime.ProcessRequest (System.Web.HttpWorkerRequest) <0x0004b>
  at Mono.WebServer.MonoWorkerRequest.ProcessRequest () <0x00083>
  at Mono.WebServer.BaseApplicationHost.ProcessRequest (Mono.WebServer.MonoWorkerRequest) <0x00083>
  at Mono.WebServer.FastCgi.ApplicationHost.ProcessRequest (Mono.WebServer.FastCgi.Responder) <0x00103>
  at (wrapper remoting-invoke-with-check) Mono.WebServer.FastCgi.ApplicationHost.ProcessRequest (Mono.WebServer.FastCgi.Responder) <IL 0x00039, 0xffffffff>
  at (wrapper xdomain-dispatch) Mono.WebServer.FastCgi.ApplicationHost.ProcessRequest (object,byte[]&,byte[]&) <0xffffffff>
  at (wrapper xdomain-invoke) Mono.WebServer.FastCgi.ApplicationHost.ProcessRequest (Mono.WebServer.FastCgi.Responder) <IL 0x000b3, 0xffffffff>
  at (wrapper remoting-invoke-with-check) Mono.WebServer.FastCgi.ApplicationHost.ProcessRequest (Mono.WebServer.FastCgi.Responder) <IL 0x00039, 0xffffffff>
  at Mono.WebServer.FastCgi.Responder.Process () [0x00060] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Responder.cs:90
  at Mono.FastCgi.ResponderRequest.Worker (object) [0x00001] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/ResponderRequest.cs:136
  at Mono.FastCgi.ResponderRequest.OnInputDataReceived (Mono.FastCgi.Request,Mono.FastCgi.DataReceivedArgs) [0x0008f] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/ResponderRequest.cs:93
  at Mono.FastCgi.Request.AddInputData (Mono.FastCgi.Record) [0x00058] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Request.cs:375
  at Mono.FastCgi.Connection.Run () [0x00298] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Connection.cs:225
  at Mono.FastCgi.Server.OnAccept (System.IAsyncResult) [0x00119] in /home/andres1304/Code/system_mono/xsp/src/Mono.WebServer.FastCgi/Server.cs:366
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void__this___object (object,intptr,intptr,intptr) <IL 0x00052, 0xffffffff>