add test (closes #5086)

tests/unit/src/unit/issues/Issue5086.hx

@@ -0,0 +1,26 @@
+package unit.issues;
+
+private class StringOutput extends haxe.io.Output {
+     var buf:StringBuf;
+
+    public function new() {
+        buf = new StringBuf();
+    }
+
+    override public function writeByte( c : Int ) : Void {
+        buf.addChar(c);
+    }
+
+    public function toString() : String {
+        return buf.toString();
+    }
+}
+
+class Issue5086 extends Test {
+    function test() {
+        var so = new StringOutput();
+        var input = "Hello.  I think Haxe is great!";
+        so.writeString(input);
+        eq(so.toString(), input);
+    }
+}