Gson.toJson: CharSequence passed to Appendable does not implement toString()

[Marcono1234]

When calling Gson.toJson(..., Appendable) and Appendable is not an instance of Writer, then Gson creates a CharSequence which does not fulfill the toString() requirements:

Returns a string containing the characters in this sequence in the same order as this sequence. The length of the string will be the length of this sequence.

Contrived example:

static class MyAppendable implements Appendable {
    private final StringBuilder stringBuilder = new StringBuilder();
    
    @Override
    public Appendable append(char c) throws IOException {
        stringBuilder.append(c);
        return this;
    }
    
    @Override
    public Appendable append(CharSequence csq) throws IOException {
        if (csq == null) {
            append("null");
        } else {
            append(csq, 0, csq.length());
        }
        return this;
    }
    
    public Appendable append(CharSequence csq, int start, int end) throws IOException {
        if (csq == null) {
            csq == "null";
        }
        
        // According to doc, toString() must return string representation
        String s = csq.toString();
        stringBuilder.append(s, start, end);
        return this;
    }
}

public static void main(String[] args) {
    MyAppendable myAppendable = new MyAppendable();
    new Gson().toJson("test", myAppendable);
    // Prints `com.` (first 4 chars of `com.google.gson.internal.Streams.AppendableWriter.CurrentWrite`)
    System.out.println(myAppendable.stringBuilder);
}

[Marcono1234]

This is actually a duplicate of #527 (though maybe this issue here can be kept open because it includes a self-contained reproduction example)?