Default $network_wide to boolean when activating plugins

Since we were not defaulting to a boolean value, the $network_wide
variable was null, when the `--network` flag was omitted.

This causes type errors when activated plugins run their activation
hooks and they expect a boolean value for the $network_wide variable,
as illustrated in issue #348.

features/plugin-activate.feature

@@ -62,6 +62,27 @@ Feature: Activate WordPress plugins
       Plugin 'user-switching' activated.
       """
 
+  Scenario: Activating a plugin with no network wide option passes down correct types
+    Given a wp-content/plugins/example-plugin.php file:
+      """
+      <?php
+      // Plugin Name: Example Plugin
+
+      function example_plugin_activate( bool $network_wide = false ) {
+        // Doesn't matter what we do here, we just need a function definition to check the type
+        return;
+      }
+
+      register_activation_hook( __FILE__, 'example_plugin_activate' );
+      """
+
+    When I run `wp plugin activate example-plugin`
+    Then STDOUT should be:
+      """
+      Plugin 'example-plugin' activated.
+      Success: Activated 1 of 1 plugins.
+      """
+
   Scenario: Not giving a slug on activate should throw an error unless --all given
     When I try `wp plugin activate`
     Then the return code should be 1

src/Plugin_Command.php

@@ -310,7 +310,7 @@ protected function get_all_items() {
 	 *     Success: Network activated 1 of 1 plugins.
 	 */
 	public function activate( $args, $assoc_args = array() ) {
-		$network_wide = Utils\get_flag_value( $assoc_args, 'network' );
+		$network_wide = Utils\get_flag_value( $assoc_args, 'network', false );
 		$all          = Utils\get_flag_value( $assoc_args, 'all', false );
 		$all_exclude  = Utils\get_flag_value( $assoc_args, 'exclude' );