fix($compile): support transcluding SVG nodes when there is no root <svg> node

Due to https://github.com/angular/angular.js/commit/f0e12ea7fea853192e4eead00b40d6041c5f914a, it is
now possible to have a directive whose template content is SVG.

However, the following issue arises:

A template like

    <replaces-with-svg-and-transcludes>
      <g>
        <circle r=50 cx=0 cy=0 />
        <text>My circle!</text>
      </g>
    </replaces-with-svg-and-transcludes>

Would not work as expected, due to the group, circle and text nodes being parsed as HTMLUnknownElements,
for not being parsed in an SVG context.

This change enables transcluded SVG content to work when parsed as non-HTML content, with the cost of
linking the transcluded elements again.