Fix LZ4 decompression on Windows 8.
Lz4DecoderStream: Replace Close() override with Dispose(bool) override as Close() is not supported in .NET 4.5 Core.  Dispose(bool) is supported everywhere.
ContentManager: Use a byte[] for LZ4 decompression as MemoryStream.GetBuffer() is not supported in .NET 4.5 Core.  Creating a MemoryStream with a byte[] shares the same buffer, so it is no more expensive.