Fix issue with is month check

Fix #3032

(cherry picked from commit 280ad6d)

Author: kylekatarnls

src/Carbon/Traits/Comparison.php

@@ -994,6 +994,13 @@ public function is(string $tester)
             return $this->year === (int) $tester;
         }
 
+        if (preg_match('/^(?:Jan|January|Feb|February|Mar|March|Apr|April|May|Jun|June|Jul|July|Aug|August|Sep|September|Oct|October|Nov|November|Dec|December)$/i', $tester)) {
+            return $this->isSameMonth(
+                $this->transmitFactory(static fn () => static::parse($tester)),
+                false,
+            );
+        }
+
         if (preg_match('/^\d{3,}-\d{1,2}$/', $tester)) {
             return $this->isSameMonth(static::parse($tester));
         }

tests/Carbon/IsTest.php

@@ -1035,6 +1035,9 @@ public function testIs()
         $this->assertTrue(Carbon::parse('2019-06-02 12:23:45')->is('Sunday'));
         $this->assertFalse(Carbon::parse('2019-06-02 12:23:45')->is('Monday'));
         $this->assertTrue(Carbon::parse('2019-06-02 12:23:45')->is('June'));
+        $this->assertTrue(Carbon::parse('2019-05-31 12:23:45')->is('May'));
+        $this->assertTrue(Carbon::parse('2019-05-31 12:23:45')->is('mAy'));
+        $this->assertFalse(Carbon::parse('2019-05-31 12:23:45')->is('April'));
         $this->assertFalse(Carbon::parse('2019-06-02 12:23:45')->is('May'));
         $this->assertTrue(Carbon::parse('2019-06-02 12:23:45')->is('12:23'));
         $this->assertFalse(Carbon::parse('2019-06-02 12:23:45')->is('12:26'));