Check headers sent before sending PHP response

If the response contents has been sent before an error occurs, PHP
triggers the warning "Cannot modify header information - headers already sent"

This change ensure that the error message is echoed, while it's impossible
to change the HTTP status code and headers.

ExceptionHandler.php

@@ -91,9 +91,11 @@ class ExceptionHandler
             $exception = FlattenException::create($exception);
         }
 
-        header(sprintf('HTTP/1.0 %s', $exception->getStatusCode()));
-        foreach ($exception->getHeaders() as $name => $value) {
-            header($name.': '.$value, false);
+        if (!headers_sent()) {
+            header(sprintf('HTTP/1.0 %s', $exception->getStatusCode()));
+            foreach ($exception->getHeaders() as $name => $value) {
+                header($name.': '.$value, false);
+            }
         }
 
         echo $this->decorate($this->getContent($exception), $this->getStylesheet($exception));

Tests/Exception/FlattenExceptionTest.php

@@ -113,7 +113,7 @@ class FlattenExceptionTest extends \PHPUnit_Framework_TestCase
 
         $this->assertEquals($exception->getMessage(), $flattened->getMessage(), 'The message is copied from the original exception.');
         $this->assertEquals($exception->getCode(), $flattened->getCode(), 'The code is copied from the original exception.');
-        $this->assertEquals(get_class($exception), $flattened->getClass(), 'The class is set to the class of the original exception');
+        $this->assertInstanceOf($flattened->getClass(), $exception, 'The class is set to the class of the original exception');
 
     }