[#465] Sanitize header name on SlickGrid view

[smotornyuk]

Change logic of sanitizing. Previously either original value
or content of element with matching selector was used as column
header.

After this change original value with applied encoding of
html entities will be used

[rufuspollock]

@smotornyuk does this avoid the security issue mentioned in #465 - i.e. does the html encoding preventing any js injection?

[amercader]

@rgrp it does. @smotornyuk patch will escape any html entities, eg given

id,<script>alert('title')</script>,dog
1,Hi,Woof
2,Hola,Guau

The header will be parsed to <script>alert('title')</script> and displayed like this (but without executing it) :

[rufuspollock]

Great and @amercader sounds like you are happy with this so I'm merging.

Thanks @smotornyuk 👍