fix: detect svgs with title as image/svg+xml intead of text/html

[sjoulbak]

Hi @jeremy 👋

I just updated marcel and noticed some failing specs on our side. After some investigation I saw that SVGs with a title did not had the context type of image/svg+xml anymore, but text/html instead. This issue is introduced after merging #74 and released at 1.0.3. The title element provides an accessible, short-text description of the SVG and so cannot be used to determine whether an element is text/html.

[sjoulbak]

@jeremy Thanks for the review! I'll undo the change at tables.rb and follow your instructions at a later moment.