Inconsistencies with Cookie value encoding

[FabianFrank]

In ktor 0.9.5 (and Cookie.kt on master as of right now https://github.com/ktorio/ktor/blob/06a0c5e70688277df32dcd6c7a6deff30561409d/ktor-http/src/io/ktor/http/Cookie.kt), there seems to be a discrepancy between what ktor allows as a raw value and the raw values it produces. The = sign is considered to not be allowed in a raw cookie value:

private val cookieCharsShouldBeEscaped = setOf(';', ',', '=', '"')
private fun Char.shouldEscapeInCookies() = this.isWhitespace() || this < ' ' || this in cookieCharsShouldBeEscaped

but when encoding a cookie value with CookieEncoding.BASE64_ENCODING a Base64 variant is chosen that will pad with =:

CookieEncoding.BASE64_ENCODING -> value.encodeBase64()

I think it would be better if ktor would either choose to allow = in cookie values in general or as an alternative use a Base64 encoding variant that does not produce characters that are disallowed in cookie values according to shouldEscapeInCookies(). Personally I think it would be best to allow = in cookie values (but not names), because a lot of existing systems are doing so and the way I understand https://tools.ietf.org/html/rfc6265#section-4.1.1 = is still allowed in cookie values. I'm happy to contribute a PR that would change the code to apply different character sets if we agree that this is the right thing to do.