Allow attributes that Adobe Illustrator export makes

[Bersman]

We have exported a SVG with Adobe and the sanitizer does not like that. It give the following errors:
There are sanitization issues with this SVG file:
Suspicious attribute 'space' in line 4
Suspicious attribute 'enable-background' in line 4

Generator is: Adobe Illustrator 19.1.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)

I wil make an PR for the enable-background attribute.
The space attribute is something different. Because it's in the list. It's looks like that XPath or something else is removing the xml: prefix when walking over the elements. Will make a separated issue for this.

[ohader]

Can you please add an example SVG snippet showing usage of enable-background? Thx

[ohader]

Spotted <svg ... enable-background="new 0 0 600 600"> in PR #65