crypto: fix default encoding regression

The default encoding is 'buffer'. When the input is a string, treat it
as 'binary'. Fixes the following assertion:

```
  node: ../src/string_bytes.cc:309: static size_t
  node::StringBytes::StorageSize(v8::Handle<v8::Value>, node::encoding):
  Assertion `0 && "buffer encoding specified but string provided"'
  failed.
```

Fixes #5482.

Suggested reviewers: @isaacs @piscisaureus