decodeWithCharset doesn't flush

[rossabaker]

In fs2-3.1.3, this test fails:

    // Found by scalacheck in old http4s implementation
    test("handle overflow") {
      if (isJVM) { // Charset not supported by Scala.js
        val cs = Charset.forName("x-ISCII91")
        val s = Stream(-36.toByte)
          .covary[Fallible]
          .through(decodeWithCharset(cs))
          .compile
          .string
        assertEquals(s, Right("ी"))
      }
    }

The output buffer is sized to one character based on chunk length and average size queried from the decoder, but the byte expands to two characters. We need to check the CoderResult for an overflow. [edit: misdiagnosis]

This is one of the rare character sets that needs to be flushed after the final input. You can find more here.