Allow `sizes` attribute in headLink (used with rel="icon")

See http://www.w3.org/TR/html5/links.html#attr-link-sizes
#6635 - switched `itemKeys` property values declaration to one-per-line for readability
#6635 - more specific type hint
#6635 - test case to prevent regressions on the 'sizes' attribute
Merge branch 'hotfix/#6635-allow-icon-sizes-for-rel-tag'

Close #6635