Base64 APIs

-- please provide the version of installed library and RN project.

0.32

-- a sample code snippet/repository is very helpful to spotting the problem.

actually it's a question about your readme, when you say

``` markdown
// here's the body you're going to send, should be a BASE64 encoded string
// (you can use "base64" APIs to make one).
```

What ""base64" APIs" are you referring to?
