Assertion ElementHasAttributes added.

Proposing, `browser.assert.elementHasAttributes( [docString,] selector, attributesHash)`
That asserts that the element contains `attribute:value` pairs specified by attributesHash.

That's mostly the effect the ‘equal‘ below already has. Using a new variable for the docstring constructed here sounds good, though, and more importantly, actually using the user-provided doc attribute in it.

Alternatively, which might be the better solution addressing both our concerns: change this to ‘attrDoc = doc ? "...as above..."‘