Do not upper case first character of headers in socket adapter

src/Client/Adapter/Socket.php

@@ -381,7 +381,7 @@ public function write($method, $uri, $httpVer = '1.1', $headers = [], $body = ''
         $request = $method . ' ' . $path . ' HTTP/' . $httpVer . "\r\n";
         foreach ($headers as $k => $v) {
             if (is_string($k)) {
-                $v = ucfirst($k) . ': ' . $v;
+                $v = $k . ': ' . $v;
             }
             $request .= $v . "\r\n";
         }

src/Client/Adapter/Test.php

@@ -132,7 +132,7 @@ public function write($method, $uri, $httpVer = '1.1', $headers = [], $body = ''
         $request = $method . ' ' . $path . ' HTTP/' . $httpVer . "\r\n";
         foreach ($headers as $k => $v) {
             if (is_string($k)) {
-                $v = ucfirst($k) . ': ' . $v;
+                $v = $k . ': ' . $v;
             }
             $request .= $v . "\r\n";
         }

test/Client/SocketTest.php

@@ -309,6 +309,24 @@ public function testAllowsZeroWrittenBytes()
         $this->_adapter->write('GET', new Uri('tcp://localhost:80/'), '1.1', [], 'test body');
     }
 
+    /**
+     * Verifies that the headers are being set as given without changing any
+     * character case.
+     */
+    public function testCaseInsensitiveHeaders()
+    {
+        $this->_adapter->connect('localhost');
+        $requestString = $this->_adapter->write(
+            'GET',
+            new Uri('tcp://localhost:80/'),
+            '1.1',
+            ['x-test-header' => 'someTestHeader'],
+            'someTestBody'
+        );
+
+        $this->assertContains('x-test-header', $requestString);
+    }
+
     /**
      * Data Providers
      */