Emitters should return boolean

This will allow us to address the issues with overloading the return
value in order to accommodate the `EmitterStack`.

Author: weierophinney

src/Emitter/EmitterInterface.php

@@ -25,6 +25,10 @@ interface EmitterInterface
      *
      * Implementations MAY raise exceptions if they are unable to emit the
      * response; e.g., if headers have already been sent.
+     *
+     * Implementations MUST return a boolean. A boolean `true` indicates that
+     * the emitter was able to emit the response, while `false` indicates
+     * it was not.
      */
-    public function emit(ResponseInterface $response) : void;
+    public function emit(ResponseInterface $response) : bool;
 }

src/Emitter/SapiEmitter.php

@@ -21,13 +21,15 @@ class SapiEmitter implements EmitterInterface
      * Emits the status line and headers via the header() function, and the
      * body content via the output buffer.
      */
-    public function emit(ResponseInterface $response) : void
+    public function emit(ResponseInterface $response) : bool
     {
         $this->assertNoPreviousOutput();
 
         $this->emitHeaders($response);
         $this->emitStatusLine($response);
         $this->emitBody($response);
+
+        return true;
     }
 
     /**

src/Emitter/SapiStreamEmitter.php

@@ -23,7 +23,7 @@ class SapiStreamEmitter implements EmitterInterface
      *
      * @param int $maxBufferLength Maximum output buffering size for each iteration
      */
-    public function emit(ResponseInterface $response, int $maxBufferLength = 8192) : void
+    public function emit(ResponseInterface $response, int $maxBufferLength = 8192) : bool
     {
         $this->assertNoPreviousOutput();
         $this->emitHeaders($response);
@@ -33,10 +33,11 @@ public function emit(ResponseInterface $response, int $maxBufferLength = 8192) :
 
         if (is_array($range) && $range[0] === 'bytes') {
             $this->emitBodyRange($range, $response, $maxBufferLength);
-            return;
+            return true;
         }
 
         $this->emitBody($response, $maxBufferLength);
+        return true;
     }
 
     /**