Fix parsing if last parameter contains '&'

Fixes #6.

src/BufferingParser.php

@@ -73,7 +73,7 @@ private function parseBody(string $body, string $boundary = null): Form
                 $fields[$field][] = $value;
             }
 
-            if (\strpos($value ?? "", "&") !== false) {
+            if (\strpos($pair[1] ?? "", "&") !== false) {
                 throw new ParseException("Maximum number of variables exceeded");
             }
 

src/BufferingParserTest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Amp\Http\Server\FormParser;
+
+use Amp\Http\Server\Driver\Client;
+use Amp\Http\Server\Request;
+use function Amp\Promise\wait;
+use League\Uri\Http;
+use PHPUnit\Framework\TestCase;
+
+class BufferingParserTest extends TestCase
+{
+    public function testIssue6()
+    {
+        $body = "foobar=" . \urlencode("&");
+        $request = new Request($this->createMock(Client::class), 'GET', Http::createFromString('/'), [], $body);
+        $form = wait((new BufferingParser)->parseForm($request));
+
+        $this->assertSame('&', $form->getValue('foobar'));
+    }
+
+}