Add shouldRejectTrailingBytes test for messages

[jtraglia]

PR Description

So I messed up in #150. I didn't notice that RlpUtil#readRlpList calls checkComplete. Sorry about that.

https://github.com/ConsenSys/discovery/blob/9bb8ad3942f81450fe54a78c66f0b61e58c45b0b/src/main/java/org/ethereum/beacon/discovery/util/RlpUtil.java#L39-L47

This PR will:

• Remove extraneous checkComplete calls.
• Add assertRoundTrip and assertRejectTrailingBytes to TestUtil.
• For each message type test class:
  • Make the decoder object static final (and uppercase its name).
  • For reusability, extract "original" message into a static final field.
  • Add new shouldRejectTrailingBytes test.
  • Replace body of shouldRoundTrip with assertRoundTrip.

Also, unrelated to the core part of this PR:

• Fix minor format issue (TAUthData -> TAuthData) in HeaderImpl.
• Fix a couple of it's vs its in comments.

[ajsutton]

LGTM. Definitely good to add these missed tests.