Support string notation for plugin options, in addition to preferred array notation.

WP has set the expectation that arguments can be specified as strings joined by ampersands. While I think it's an unnecessarily-confusing way of specifying arguments, some users seem to like it.

See http://wordpress.org/support/topic/unable-to-customise-plugin-behaviour-in-non-widget-implementation.

tag-dropdown-widget.php

@@ -257,7 +257,7 @@ class taxonomy_dropdown_widget_plugin {
 
 	/**
 	 * Sanitize plugin options
-	 * @param array $options
+	 * @param array|string $options
 	 * @uses taxonomy_exists
 	 * @uses sanitize_text_field
 	 * @uses absint
@@ -265,6 +265,12 @@ class taxonomy_dropdown_widget_plugin {
 	 * @return array
 	 */
 	public function sanitize_options( $options ) {
+		// WP supports strings for arguments in place of arrays.
+		// Match the expectation, though arrays are preferred.
+		if ( is_string( $options ) ) {
+			wp_parse_str( $options, $options );
+		}
+
 		$options_sanitized = array(
 			'hide_empty'  => true,
 			'post_counts' => false,