web_renderjs: initialise web script content type to application/javascript

https://tools.ietf.org/html/rfc4329#section-7 states that text/javascript is obsolete.

Doesn't really matter in the end because WebPage_viewAsWeb forces the content type.

bt5/erp5_web_renderjs_ui/SkinTemplateItem/portal_skins/erp5_web_renderjs/WebScript_init.py

-context.setContentType('text/javascript')
+context.setContentType('application/javascript')