[wcj_currency_select_drop_down_list] issue and fix

Upgraded from 7.2.3 to 7.5.0 (latest available at booster.io) and the currency switcher shortcode [wcj_currency_select_drop_down_list] stopped working.

The reason is in the form element being stripped from the submit() code.

This is the code in 7.2.3:

<div class="sw-currency">
<span class="sw-label">Preferred Currency</span><br>
<select name="wcj-currency" id="wcj-currency-select" style="" class="" onchange="this.form.submit()">
<option value="EUR">€ : Euro</option>
<option value="USD" selected="selected">$ : US dollar</option>
<option value="GBP">? : Pound sterling</option>
<option value="AUD">A$ : Australian dollar</option>
<option value="CAD">C$ : Canadian dollar</option>
</select>
<input type="hidden" id="wcj-currency-nonce" name="wcj-currency-nonce" value="b9a9e3f79d">
<input type="hidden" name="_wp_http_referer" value="/checkout/">
</div>

This is the code in 7.5.0:

<div class="sw-currency">
<span class="sw-label">Preferred Currency</span><br>
<select name="wcj-currency" id="wcj-currency-select" class="">
<option value="EUR">€ : Euro</option>
<option value="USD">$ : US dollar</option>
<option value="GBP" selected="selected">? : Pound sterling</option>
<option value="AUD">A$ : Australian dollar</option>
<option value="CAD">C$ : Canadian dollar</option>
</select>
<input type="hidden" id="wcj-currency-nonce" name="wcj-currency-nonce" value="b9a9e3f79d">
<input type="hidden" name="_wp_http_referer" value="/checkout/">
</div>

Note how the wcj-currency-select line is being stripped from onchange="this.form.submit()"

The culprit I was able to locate is in \booster-plus-for-woocommerce\includes\classes\class-wcj-shortcodes.php line 405:

$result = ( is_string( $result ) ? wp_kses_post( $result ) : $result );

The wp_kses_post function is being too strong here. Commenting out the line above and the issue gets fixed.