Removes Content-Length injection from SAPI stream emitters

Per #251, this patch removes Content-Length injection from SAPI stream
emitters. It also renames the `checkForPreviousOutput()` method to
`assertNoPreviousOutput()` to make it more clear that the method is a
no-op under proper usage.

The `assertNoPreviousOutput()` method also borrows an idea from the
comment to the issue, indicating we can check for (1) a non-zero output
buffer, and (2) use `ob_get_length()` to determine if it has content.

src/Response/SapiEmitter.php

@@ -26,9 +26,7 @@ class SapiEmitter implements EmitterInterface
      */
     public function emit(ResponseInterface $response)
     {
-        $this->checkForPreviousOutput();
-
-        $response = $this->injectContentLength($response);
+        $this->assertNoPreviousOutput();
 
         $this->emitStatusLine($response);
         $this->emitHeaders($response);

src/Response/SapiEmitterTrait.php

@@ -17,40 +17,21 @@ trait SapiEmitterTrait
     /**
      * Checks to see if content has previously been sent.
      *
-     * If either headers have been sent, or the current output buffer contains
-     * content, raises an exception.
+     * If either headers have been sent or the output buffer contains content,
+     * raises an exception.
      *
      * @throws RuntimeException if headers have already been sent.
-     * @throws RuntimeException if the current output buffer is not empty.
+     * @throws RuntimeException if output is present in the output buffer.
      */
-    private function checkForPreviousOutput()
+    private function assertNoPreviousOutput()
     {
         if (headers_sent()) {
             throw new RuntimeException('Unable to emit response; headers already sent');
         }
-        $bufferContents = ob_get_contents();
-        if (! empty($bufferContents)) {
-            throw new RuntimeException('Output has been emitted previously; cannot emit response: ' . $bufferContents);
-        }
-    }
 
-    /**
-     * Inject the Content-Length header if is not already present.
-     *
-     * @param ResponseInterface $response
-     * @return ResponseInterface
-     */
-    private function injectContentLength(ResponseInterface $response)
-    {
-        if (! $response->hasHeader('Content-Length')) {
-            // PSR-7 indicates int OR null for the stream size; for null values,
-            // we will not auto-inject the Content-Length.
-            if (null !== $response->getBody()->getSize()) {
-                return $response->withHeader('Content-Length', (string) $response->getBody()->getSize());
-            }
+        if (ob_get_level() > 0 && ob_get_length() > 0) {
+            throw new RuntimeException('Output has been emitted previously; cannot emit response: ' . $bufferContents);
         }
-
-        return $response;
     }
 
     /**

src/Response/SapiStreamEmitter.php

@@ -28,10 +28,7 @@ class SapiStreamEmitter implements EmitterInterface
      */
     public function emit(ResponseInterface $response, $maxBufferLength = 8192)
     {
-        $this->checkForPreviousOutput();
-
-        $response = $this->injectContentLength($response);
-
+        $this->assertNoPreviousOutput();
         $this->emitStatusLine($response);
         $this->emitHeaders($response);
 

test/Response/AbstractEmitterTest.php

@@ -44,7 +44,6 @@ public function testEmitsResponseHeaders()
         ob_end_clean();
         $this->assertContains('HTTP/1.1 200 OK', HeaderStack::stack());
         $this->assertContains('Content-Type: text/plain', HeaderStack::stack());
-        $this->assertContains('Content-Length: 8', HeaderStack::stack());
     }
 
     public function testEmitsMessageBody()

test/ServerTest.php

@@ -251,7 +251,6 @@ public function testEmitsHeadersWithMultipleValuesMultipleTimes()
      */
     public function testHeaderOrderIsHonoredWhenEmitted($stack)
     {
-        array_pop($stack); // ignore "Content-Length" automatically set by the response emitter
         $header = array_pop($stack);
         $this->assertContains(
             'Set-Cookie: bar=baz; expires=Wed, 8 Oct 2014 10:30; path=/foo/bar; domain=example.com',