Header parsing improvements

* Made headers public readonly in HeadersFrame
* Fetching headers by value implemented
* Initial tests for HeaderBag

tests/Header/HeaderBagTest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace NoccyLabs\React\Http2\Header;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+
+#[CoversClass(HeaderBag::class)]
+#[CoversClass(HeaderPacker::class)]
+class HeaderBagTest extends \PHPUnit\Framework\TestCase
+{
+
+    public function testHeadersInExamplesFromRfc7541()
+    {
+        $packer = new HeaderPacker();
+
+        $in = "\x82\x86\x84\x41\x8c\xf1\xe3\xc2\xe5\xf2\x3a\x6b\xa0\xab\x90\xf4\xff";
+        $expect = new HeaderBag([
+            ':method' => 'GET',
+            ':scheme' => 'http',
+            ':path' => '/',
+            ':authority' => 'www.example.com'
+        ]);
+        $out = $packer->unpackHeaders($in);
+
+        $this->assertEquals('GET', $out->getHeaderLine(':method'));
+        $this->assertEquals('http', $out->getHeaderLine(':scheme'));
+        $this->assertEquals('/', $out->getHeaderLine(':path'));
+        $this->assertEquals('www.example.com', $out->getHeaderLine(':authority'));
+
+    }
+
+    
+}