Fix exceptions caused by blob.content including `\n`

octotree stopped in some repositories.
e.g.: https://github.com/ReactiveCocoa/ReactiveCocoa

Some blob returns base64 content including `\n`.
This cause exception in `atob()` and stop octotree.
The document does not mention about it.
https://developer.github.com/v3/git/blobs/#get-a-blob