Make FrozenClock mutable through setter

Because the FrozenClock is often injected in other objects during
testing, being able to change the current time on the FrozenClock
without reinjecting it in the object under test is quite useful.

src/FrozenClock.php

@@ -18,6 +18,11 @@ public function __construct(DateTimeImmutable $now)
         $this->now = $now;
     }
 
+    public function setTo(DateTimeImmutable $now): void
+    {
+        $this->now = $now;
+    }
+
     public function now(): DateTimeImmutable
     {
         return $this->now;

test/FrozenClockTest.php

@@ -23,4 +23,23 @@ public function nowShouldReturnAlwaysTheSameObject()
         self::assertSame($now, $clock->now());
         self::assertSame($now, $clock->now());
     }
+
+    /**
+     * @test
+     *
+     * @covers \Lcobucci\Clock\FrozenClock::setTo
+     * @uses   \Lcobucci\Clock\FrozenClock::__construct()
+     * @uses   \Lcobucci\Clock\FrozenClock::now
+     */
+    public function nowSetChangesTheObject()
+    {
+        $oldNow = new DateTimeImmutable();
+        $clock  = new FrozenClock($oldNow);
+
+        $newNow = new DateTimeImmutable();
+        $clock->setTo($newNow);
+
+        self::assertNotSame($oldNow, $clock->now());
+        self::assertSame($newNow, $clock->now());
+    }
 }