FIX Template discovery on themed Layout templates

Was failing when 'main' template only exists in theme,
but 'Layout' template only exists in module.

core/manifest/TemplateManifest.php

@@ -115,11 +115,11 @@ class SS_TemplateManifest {
 		if ($this->project && isset($candidates[$this->project])) {
 			$found = $candidates[$this->project];
 		} else if ($theme && isset($candidates['themes'][$theme])) {
-			$found = $candidates['themes'][$theme];
+			$found = array_merge($candidates, $candidates['themes'][$theme]);
 		} else {
-			unset($candidates['themes']);
 			$found = $candidates;
 		}
+		if(isset($found['themes'])) unset($found['themes']);
 
 		return $found;
 	}

tests/core/manifest/TemplateLoaderTest.php

@@ -42,6 +42,13 @@ class TemplateLoaderTest extends SapphireTest {
 		);
 
 		$this->assertEquals($expectCustomPage, $loader->findTemplates(array('CustomPage', 'Page')));
+
+		// 'main' template only exists in theme, and 'Layout' template only exists in module
+		$expectCustomThemePage = array(
+			'main'   => "$base/themes/theme/templates/CustomThemePage.ss",
+			'Layout' => "$base/module/templates/Layout/CustomThemePage.ss"
+		);
+		$this->assertEquals($expectCustomThemePage, $loader->findTemplates(array('CustomThemePage', 'Page'), 'theme'));
 	}
 
 	public function testFindTemplatesApplicationOverridesModule() {

tests/core/manifest/TemplateManifestTest.php

@@ -47,6 +47,13 @@ class TemplateManifestTest extends SapphireTest {
 			'subfolder' => array(
 				'main' => "{$this->base}/module/subfolder/templates/Subfolder.ss"
 			),
+			'customthemepage' => array (
+			  'Layout' => "{$this->base}/module/templates/Layout/CustomThemePage.ss",
+			  'themes' =>
+			  array(
+			    'theme' => array('main' => "{$this->base}/themes/theme/templates/CustomThemePage.ss",)
+			  )
+			),
 			'include' => array('themes' => array(
 				'theme' => array(
 					'Includes' => "{$this->base}/themes/theme/templates/Includes/Include.ss"