Improve PumpReader surrogate char handling

[Marcono1234]

Addresses the additional notes in #658 and #659

Improves PumpReader surrogate char handling by making sure that the reader blocks when an incomplete surrogate pair (i.e. only a high surrogate char) is encountered.

Additionally changes the reading logic to consume pending data even if the reader has been closed. Otherwise it would be impossible to read a trailing high surrogate char because CharsetEncoder.encode was previously always called with endOfInput=false and therefore would have never encoded that trailing high surrogate char at the end of the stream (maybe that was intended?).

The added AssertionErrors should not be reachable, even for invalid / incomplete input, also due to #716.