Throw error in theme fetcher on non-lowercase slugs

Author: schlessera

features/plugin-activate.feature

@@ -80,3 +80,17 @@ Feature: Activate WordPress plugins
       """
       Success: No plugins installed.
       """
+
+  Scenario: Using a non-lowercase slug should not find a plugin
+    When I try `wp plugin activate Akismet`
+    Then STDOUT should be empty
+    And STDERR should contain:
+      """
+      The 'Akismet' plugin could not be found.
+      """
+
+    When I run `wp plugin activate akismet`
+    Then STDOUT should contain:
+      """
+      Plugin 'akismet' activated.
+      """

features/theme.feature

@@ -517,3 +517,19 @@ Feature: Manage WordPress themes
       """
       Error: Can't find the requested theme's version 1.4.2 in the WordPress.org theme repository (HTTP code 404).
       """
+
+  Scenario: Using a non-lowercase slug should not find a theme
+    Given a WP install
+
+    When I try `wp theme activate TwentySixteen`
+    Then STDOUT should be empty
+    And STDERR should contain:
+      """
+      The 'TwentySixteen' theme could not be found.
+      """
+
+    When I run `wp theme activate twentysixteen`
+    Then STDOUT should be:
+      """
+      Success: Switched to 'Twenty Sixteen' theme.
+      """

src/WP_CLI/Fetchers/Theme.php

@@ -14,11 +14,16 @@ class Theme extends Base {
 
 	/**
 	 * Get a theme object by name
-	 * 
+	 *
 	 * @param string $name
 	 * @return object|false
 	 */
 	public function get( $name ) {
+		// To avoid later issues, we force slugs to be lowercase.
+		if( strtolower( $name ) !== $name ) {
+			return false;
+		}
+
 		$theme = wp_get_theme( $name );
 
 		if ( !$theme->exists() ) {