Add missing cast to SoftwareSerial::peek() when returning buffered value

[sandeepmistry]

Resolves #2858.

Prior to this, read and peek would return different values if the byte value was greater than 127.

For example, if the buffered byte was 180, read would return 180, but peek would return -76.

[ArduinoBot]

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[matthijskooijman]

I wonder if it would not be better to change the buffer to an uint8_t instead, removing the need for these casts. I don't see any reason for the buffer to use char, as it does now (even more because char is ambiguous in signedness, if is must be signed it should at least use signed char or int8_t).

[sandeepmistry]

Yes, I agree. It can be changed to unsigned char (this is what HardwareSerial uses) or uint8_t. This will also slightly clean up the read method.

I'll work on this, and then update the PR.

[sandeepmistry]

@matthijskooijman I've pushed the changes, if things look good, I will go ahead and squash the commits.

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

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⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4669-BUILD-523-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4669-BUILD-523-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4669-BUILD-523-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.