Moved body from string to buffer

**Major release** (due to alteration of `forwardRequest` and `normalizeFn` data)

**Depends on #27**

In order to handle binary content, we have decided to maintain the single file format with conditional logic to serialize any UTF8 incompatible content (e.g. binary data) via base64. The fixtures themselves are backwards compatible but our interface is not.

In this PR:

- Remove stringifcation of `Buffer` in `lib/message`
- Serialize/deserialize body with `utf8` or `base64` encoding (depends on if it gets mangled)
- Added regression test for binary content (both in-memory and on-disk)

Fixes #17