Move "reuse part of the buffer" logic into UploadBuffer
Move "reuse part of the buffer" logic into UploadBuffer

jskeet/google-api-dotnet-client,googleapis/google-api-dotnet-client,jskeet/google-api-dotnet-client,googleapis/google-api-dotnet-client,jskeet/google-api-dotnet-client,googleapis/google-api-dotnet-client