Allow underscore character in Uri host

I don't understand why this validity check has be added.
Has per rfc 2181
(https://datatracker.ietf.org/doc/html/rfc2181#section-11), underscore
are valid character to use in an uri host.

For my specific usage, it broke for requests using docker internal
hostnames.

added test to prevent regression on URI containing underscore in host

src/Message/Uri.php

@@ -55,7 +55,7 @@ public function __construct($uri)
         }
         // @codeCoverageIgnoreEnd
 
-        if ($parts === false || (isset($parts['scheme']) && !\preg_match('#^[a-z]+$#i', $parts['scheme'])) || (isset($parts['host']) && \preg_match('#[\s_%+]#', $parts['host']))) {
+        if ($parts === false || (isset($parts['scheme']) && !\preg_match('#^[a-z]+$#i', $parts['scheme'])) || (isset($parts['host']) && \preg_match('#[\s%+]#', $parts['host']))) {
             throw new \InvalidArgumentException('Invalid URI given');
         }
 
@@ -173,7 +173,7 @@ public function withHost($host)
             return $this;
         }
 
-        if (\preg_match('#[\s_%+]#', $host) || ($host !== '' && \parse_url('http://' . $host, \PHP_URL_HOST) !== $host)) {
+        if (\preg_match('#[\s%+]#', $host) || ($host !== '' && \parse_url('http://' . $host, \PHP_URL_HOST) !== $host)) {
             throw new \InvalidArgumentException('Invalid URI host given');
         }
 

tests/Message/UriTest.php

@@ -120,6 +120,9 @@ public static function provideValidUris()
             ),
             array(
                 'http://user%20name:pass%20word@localhost/path%20name?query%20name#frag%20ment'
+            ),
+            array(
+                'http://docker_container/'
             )
         );
     }
@@ -338,6 +341,16 @@ public function testWithHostReturnsNewInstanceWhenHostIsChanged()
         $this->assertEquals('localhost', $uri->getHost());
     }
 
+    public function testWithHostReturnsNewInstanceWhenHostIsChangedWithUnderscore()
+    {
+        $uri = new Uri('http://localhost');
+
+        $new = $uri->withHost('docker_container');
+        $this->assertNotSame($uri, $new);
+        $this->assertEquals('docker_container', $new->getHost());
+        $this->assertEquals('localhost', $uri->getHost());
+    }
+
     public function testWithHostReturnsNewInstanceWithHostToLowerCaseWhenHostIsChangedWithUpperCase()
     {
         $uri = new Uri('http://localhost');