Move loader cascading into the loadPartial method.

This means the `partialsLoader` property is never set to the default loader, fixing things like `setPartials`, but template loading cascades as per the spec.

Add test coverage for partial cascading.

src/Mustache/Engine.php

@@ -226,7 +226,7 @@ class Mustache_Engine
     public function getPartialsLoader()
     {
         if (!isset($this->partialsLoader)) {
-            $this->partialsLoader = $this->getLoader();
+            $this->partialsLoader = new Mustache_Loader_ArrayLoader;
         }
 
         return $this->partialsLoader;
@@ -492,13 +492,21 @@ class Mustache_Engine
     public function loadPartial($name)
     {
         try {
-            return $this->loadSource($this->getPartialsLoader()->load($name));
-        } catch (InvalidArgumentException $e) {
+            if (isset($this->partialsLoader)) {
+                $loader = $this->partialsLoader;
+            } elseif (isset($this->loader) && !$this->loader instanceof Mustache_Loader_StringLoader) {
+                $loader = $this->loader;
+            } else {
+                throw new Mustache_Exception_UnknownTemplateException($name);
+            }
+
+            return $this->loadSource($loader->load($name));
+        } catch (Mustache_Exception_UnknownTemplateException $e) {
             // If the named partial cannot be found, log then return null.
             $this->log(
                 Mustache_Logger::WARNING,
                 'Partial not found: "{name}"',
-                array('name' => $name)
+                array('name' => $e->getTemplateName())
             );
         }
     }

test/Mustache/Test/EngineTest.php

@@ -104,7 +104,7 @@ class Mustache_Test_EngineTest extends PHPUnit_Framework_TestCase
         $mustache->setLoader($loader);
         $this->assertSame($loader, $mustache->getLoader());
 
-        $this->assertSame($loader, $mustache->getPartialsLoader());
+        $this->assertNotSame($loader, $mustache->getPartialsLoader());
         $mustache->setPartialsLoader($loader);
         $this->assertSame($loader, $mustache->getPartialsLoader());
 
@@ -240,6 +240,29 @@ class Mustache_Test_EngineTest extends PHPUnit_Framework_TestCase
         $mustache->setLogger(new StdClass);
     }
 
+    public function testLoadPartialCascading()
+    {
+        $loader = new Mustache_Loader_ArrayLoader(array(
+            'foo' => 'FOO',
+        ));
+
+        $mustache = new Mustache_Engine(array('loader' => $loader));
+
+        $tpl = $mustache->loadTemplate('foo');
+
+        $this->assertSame($tpl, $mustache->loadPartial('foo'));
+
+        $mustache->setPartials(array(
+            'foo' => 'f00',
+        ));
+
+        // setting partials overrides the default template loading fallback.
+        $this->assertNotSame($tpl, $mustache->loadPartial('foo'));
+
+        // but it didn't overwrite the original template loader templates.
+        $this->assertSame($tpl, $mustache->loadTemplate('foo'));
+    }
+
     public function testPartialLoadFailLogging()
     {
         $name     = tempnam(sys_get_temp_dir(), 'mustache-test');