Fix serving files (#531)

Author: adam-fowler

Benchmarks/Benchmarks/Router/RouterBenchmarks.swift

@@ -128,6 +128,7 @@ func routerBenchmarks() {
                 for try await buffer in request.body {
                     try await writer.write(buffer)
                 }
+                try await writer.finish(nil)
             })
         }
     }

Sources/Hummingbird/Files/FileIO.swift

@@ -123,6 +123,7 @@ public struct FileIO: Sendable {
                     fileOffset = range.index(fileOffset, offsetBy: bytesToRead)
                     try await writer.write(buffer)
                 }
+                try await writer.finish(nil)
             }
         }
     }