Updates direct message creation to impose a limit

Per https://dev.twitter.com/rest/reference/post/direct_messages/events/new,
DM text can be no more than 10k characters in length.

library/ZendService/Twitter/Twitter.php

@@ -479,12 +479,18 @@ public function directMessagesNew($user, $text)
         $path = 'direct_messages/new';
 
         $len = iconv_strlen($text, 'UTF-8');
-        if (0 == $len) {
+        if (0 === $len) {
             throw new Exception\InvalidArgumentException(
                 'Direct message must contain at least one character'
             );
         }
 
+        if (10000 < $len) {
+            throw new Exception\InvalidArgumentException(
+                'Direct message must be no more than 10000 characters'
+            );
+        }
+
         $params         = $this->createUserParameter($user, []);
         $params['text'] = $text;
         $response       = $this->post($path, $params);

tests/ZendService/Twitter/TwitterTest.php

@@ -87,6 +87,16 @@ protected function stubOAuthClient(
         return $client->reveal();
     }
 
+    public function stubHttpClientInitialization()
+    {
+        $client = $this->prophesize(OAuthClient::class);
+        $client->setHeaders(['Accept-Charset' => 'ISO-8859-1,utf-8'])->will([$client, 'reveal']);
+        $client->resetParameters()->will([$client, 'reveal']);
+        $client->clearCookies()->will([$client, 'reveal']);
+        $client->getCookies()->willReturn([]);
+        return $client->reveal();
+    }
+
     public function testRateLimitHeaders()
     {
         $rateLimits = [
@@ -794,4 +804,23 @@ public function testAdapterAlwaysReachableIfSpecified($config, $adapter)
         $twitter = new Twitter\Twitter($config);
         $this->assertSame($adapter, $twitter->getHttpClient()->getAdapter());
     }
+
+    public function testDirectMessagesNewRaisesExceptionForEmptyMessage()
+    {
+        $twitter = new Twitter\Twitter();
+        $twitter->setHttpClient($this->stubHttpClientInitialization());
+        $this->expectException(Twitter\Exception\InvalidArgumentException::class);
+        $this->expectExceptionMessage('at least one char');
+        $twitter->directMessagesNew('twitter', '');
+    }
+
+    public function testDirectMessagesNewRaisesExceptionForTooLongOfMessage()
+    {
+        $twitter = new Twitter\Twitter();
+        $twitter->setHttpClient($this->stubHttpClientInitialization());
+        $text = str_pad('', 10001, 'X');
+        $this->expectException(Twitter\Exception\InvalidArgumentException::class);
+        $this->expectExceptionMessage('no more than 10000 char');
+        $twitter->directMessagesNew('twitter', $text);
+    }
 }