Move isGsm7 to static for other libraries (#359)

src/SMS/Message/SMS.php

@@ -30,9 +30,9 @@ public function __construct(string $to, string $from, protected string $message,
         $this->setType($type);
     }
 
-    public function isGsm7(): bool
+    public static function isGsm7(string $message): bool
     {
-        return (bool)preg_match(self::GSM_7_PATTERN, $this->getMessage());
+        return (bool)preg_match(self::GSM_7_PATTERN, $message);
     }
 
     public function getContentId(): string
@@ -59,14 +59,14 @@ public function setEntityId(string $id): self
 
     public function getErrorMessage(): ?string
     {
-        if ($this->getType() === 'unicode' && $this->isGsm7()) {
+        if ($this->getType() === 'unicode' && self::isGsm7($this->getMessage())) {
             $this->setErrorMessage("You are sending a message as `unicode` when it could be `text` or a
             `text` type with unicode-only characters. This could result in increased billing - 
             See https://developer.vonage.com/messaging/sms for details, or email [email protected] if you have any 
             questions.");
         }
 
-        if ($this->getType() === 'text' && ! $this->isGsm7()) {
+        if ($this->getType() === 'text' && ! self::isGsm7($this->getMessage())) {
             $this->setErrorMessage("You are sending a message as `text` when contains unicode only 
             characters. This could result in encoding problems with the target device or increased billing - See 
             https://developer.vonage.com/messaging/sms for details, or email [email protected] if you have any 

test/SMS/Message/SMSTest.php

@@ -173,10 +173,9 @@ public function testDLTInfoDoesNotAppearsWhenNotSet(): void
      * @dataProvider unicodeStringDataProvider
      * @return void
      */
-    public function testGsm7Identification(string $message, bool $expectedGsm7)
+    public function testGsm7Identification(string $message, bool $expectedGsm7): void
     {
-        $sms = new SMS('16105551212', '16105551212', $message);
-        $this->assertEquals($expectedGsm7, $sms->isGsm7());
+        $this->assertEquals($expectedGsm7, SMS::isGsm7($message));
     }
 
     public function unicodeStringDataProvider(): array