…ecoder updates.

This commit applies [1] to the Play's cookie encoder/decoder implementation.

Netty was update cookie encoder/decoder at 2015/4/14 [2] and 2016/5/19 [1].
And Play was update cookie encoder/decoder at 2015/5/11. So, [1] is not applied to Play yet.

[1] is `Drop broken DefaultCookie name validation`.
Detail is discussed in issue [4] (`DefaultCookie's own name validation doesn't conform to RFC6265 playframework#4999`) .

I think, current Play's DefaultCookie has the same issue.

> it doesn't conform to RFC6265, eg seperators such as @ are allowed
> it's redundant with the checks performed in CookieEncoder/Decoder in STRICT mode (those are RFC6265 compliant)

And, RFC6265 accepts, cookie-name which starts with `$` [5] [6]

```
cookie-name       = token

token          = 1*<any CHAR except CTLs or separators>
separators     = "(" | ")" | "<" | ">" | "@"
                | "," | ";" | ":" | "\" | <">
                | "/" | "[" | "]" | "?" | "="
                | "{" | "}" | SP | HT
```

However, current play implementation throws an exception at [7].
So, I think applying [1] is suitable for better RFC following.

[1]: netty/netty@1d9c58b
[2]: netty/netty@97d871a
[3]: playframework@663dadb
[4]: netty/netty#4999
[5]: https://tools.ietf.org/html/rfc6265#section-4.1.1
[6]: https://tools.ietf.org/html/rfc2616#section-2.2
[7]: https://github.com/playframework/playframework/blob/2.5.4/framework/src/play-netty-utils/src/main/java/play/core/netty/utils/DefaultCookie.java#L61-L63