[whatwg/encoding] Concatenating two ISO-2022-JP outputs from a conforming encoding doesn't result in conforming input (#115)

Encodings other than ISO-2022-JP have the property that if you concatenate two outputs from a conforming encoder and decode them together, you get the same result as when decoding them separately and then concatenating.

ISO-2022-JP lacks this property, because despite the encoder making an effort into this direction my emitting a transition to the ASCII state at the end, if the next segment being concatenated starts with a transition to a non-ASCII state, the concatenation results in zero ASCII bytes between two escapes.

Is there a strong reason for treating a transition to the ASCII state immediately followed by another escape as non-conforming? Or put the other way, what purpose does the transition to the ASCII state at the end of encode serve if not achieving the above-mentioned concatenation property that other encodings have?

[This is relevant to RFC 2047 header decoding](https://bugzilla.mozilla.org/show_bug.cgi?id=1374149).

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