Fixed binary requests handling

[ABeltramo]

Hello there! 👋

I saw that the library wasn't working with binary (byte arrays) payloads, while the underlying NanoHTTPD does support them (kinda, see below), so here's a pull request to fix it!

Other minor changes:

• Changed a few single branch if to when
• Moved a couple of docstrings in the right place (before the arguments)
• Added a simple unit test for this new capability

Disclaimer

While this is enough to fix the exception, and it'll work easily for a roundtrip of String -> Bytes -> String it can easily break when doing Bytes -> String -> Bytes for two reasons:

• Some bytes can't be represented in the default UTF-8 encoding
  • This can be fixed by using ISO-8859-1 instead, see stackoverflow#15925458
• Unfortunately, not only it's always casted as a String, but it's also trimmed, here (when calling (.parseBody session body-map)).
  • This may result in a different byte array depending on the payload.

For this problematic payloads I couldn't find a way to properly check the request payload, the response was correctly returned as a byte array instead; no problem there.

[johanhaleby]

Really awesome! Great work :) I'll bring it in and make a new release.