Fix OpeningHours::simplifiedExpression not really being const

Reading the code I realized it was modifying the instance internally,
which isn't what one would expect from a const method.

autotests/parsertest.cpp

@@ -394,6 +394,8 @@ private Q_SLOTS:
         QVERIFY(oh.error() != OpeningHours::SyntaxError);
         QCOMPARE(oh.normalizedExpression(), expectedOutput);
         QCOMPARE(oh.simplifiedExpression(), expectedSimplifiedOutput);
+        // verify that simplifiedExpression() doesn't alter `oh`
+        QCOMPARE(oh.normalizedExpression(), expectedOutput);
 
         // verify the expressions we generate are parsed correctly as well
         OpeningHours oh2(oh.normalizedExpression());

src/lib/openinghours.cpp

@@ -432,8 +432,9 @@ QByteArray OpeningHours::normalizedExpression() const
 
 QByteArray OpeningHours::simplifiedExpression() const
 {
-    d->simplify();
-    return normalizedExpression();
+    OpeningHours copy(normalizedExpression());
+    copy.d->simplify();
+    return copy.normalizedExpression();
 }
 
 QString OpeningHours::normalizedExpressionString() const