Merge branch '3.4' into 4.1

* 3.4:
  Change button_widget class to btn-primary
  [Dotenv] dont use getenv() to read SYMFONY_DOTENV_VARS
  [HttpFoundation] Fixed PHP doc of ParameterBag::getBoolean
  [HttpFoundation] replace any preexisting Content-Type headers

ParameterBag.php

@@ -174,7 +174,7 @@ public function getInt($key, $default = 0)
      * Returns the parameter value converted to boolean.
      *
      * @param string $key     The parameter key
-     * @param mixed  $default The default value if the parameter key does not exist
+     * @param bool   $default The default value if the parameter key does not exist
      *
      * @return bool The filtered value
      */

Response.php

@@ -330,8 +330,9 @@ public function sendHeaders()
 
         // headers
         foreach ($this->headers->allPreserveCaseWithoutCookies() as $name => $values) {
+            $replace = 0 === strcasecmp($name, 'Content-Type');
             foreach ($values as $value) {
-                header($name.': '.$value, false, $this->statusCode);
+                header($name.': '.$value, $replace, $this->statusCode);
             }
         }