Add support for multiple matches

[juank-pa]

In theory the assert_jquery_method supports multiple matches for the same selector/function. But this was not working because of how Nokogiri was used. The algorithm was trying to add a tag as a root HTML element multiple times raising the "Document already has a root node" error.

At the beginning I tried to fix the code so that we add the actually parsed children to the html/body tag but then I preferred to use the more flexible fragments.

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[juank-pa]

I preferred this over Nokogiri::HTML::DocumentFragment.new because I woudl have been forced to pass an already existing HTML Document. And also preferred it over Nokogiri::HTML::DocumentFragment.parse to avoid the empty string parameter that would obscure the intention of this line of code.