URL/text comparison should be case-insensitive for hostnames.

MM shows actual URLs in a small pop-up when rolling over <a> tags in HTML mail AND when rolling over an URL in plaintext mail. When the content of the <a> tag does not match the src attribute, that pop-up is red. However, the matching is apparently being done by case-squashing the URL (or at least the hostname part) in the either the src attribute or the content of the <a> tag but not both, such that a partially capitalized hostname part in a URL causes a bogus mismatch (i.e. since DNS is case-insensitive.)

This causes a particularly notable false mismatch with plaintext mail containing URLs with capitalized hostnames. In that case it isn't actually possible for there to be a functional mismatch, as there is only a detected URL, but MM shows one.