Call toString on args to match browser behavior.

Browsers appear to call .toString on btoa args. This package has unpredictable behavior if you pass it a non-string:

````
> $ node
> var btoa = require("btoa");
undefined
> btoa((Date.now() * Math.random()).toString())
'ODczNzg4ODA3MjY3LjYwOTE='
> btoa(Date.now() * Math.random())
FATAL ERROR: CALL_AND_RETRY_0 Allocation failed - process out of memory
> btoa(Date.now() * Math.random())
FATAL ERROR: v8::Object::SetIndexedPropertiesToExternalArrayData() length exceeds max acceptable value
> btoa((Date.now() * Math.random()))
''
````

If the number is small(?), the output is wrong:

````
// browser:
> btoa(1234)
'MTIzNA=='
// current node-btoa
> btoa(1234)
'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'
````