Customize: Prevent outputting `value` attribute if already supplied among `input_attrs`.

This allows for `input[type=button]` controls to be added without producing illegal HTML.

Amends [41740].
See #30738, #33085.


git-svn-id: https://develop.svn.wordpress.org/trunk@41822 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-control.php

@@ -649,7 +649,9 @@ class WP_Customize_Control {
 					type="<?php echo esc_attr( $this->type ); ?>"
 					<?php echo $describedby_attr; ?>
 					<?php $this->input_attrs(); ?>
-					value="<?php echo esc_attr( $this->value() ); ?>"
+					<?php if ( ! isset( $this->input_attrs['value'] ) ) : ?>
+						value="<?php echo esc_attr( $this->value() ); ?>"
+					<?php endif; ?>
 					<?php $this->link(); ?>
 					/>
 				<?php