FileTransfer did not abort on ios

This fix works for me - don't know much about Objective C but in ["cancelTransfer"-method](https://github.com/bastibeckr/cordova-plugin-file-transfer/blob/58fc57dc9b22f13bd5e33e3b180a0a0f9f6bf428/src/ios/CDVFileTransfer.m#L573-578) the connection-Object was NULL. Hope this helps.