protocol: non-space whitespace does not round trip correctly via Encode/StreamParser

There are special cases in the code to escape tab, newline, carriage-return and form-feed when encoding line-protocol values.
It seems that those cases aren't respected correctly when decoding.

For example, [this code](https://play.golang.org/p/Z0t8bVOoHLs) fails because it can't parse the line that has been encoded.

[This code](https://play.golang.org/p/YpHiH0Ne7Vj) fails because the value has been silently changed when round tripping.

I appreciate that the encoding conventions probably can't change, but perhaps we could consider returning an error when the encoded values include these characters, to avoid this potential trip hazard.