no-mergeable-selectors can't handle media-queries with functions

[thasmo]

When using a media-query with a function, like @media(min-width: break('large')) the no-mergeable-selectors rule has a false positive, because it's evaluating the rule to @media(min-width: null) for all break function calls and thus throws a warning for media-queries using different values in the break function.

This seems to be related to #703 which probably included a fix for variables but not for functions.

[DanPurdy]

Thanks for the issue @thasmo I'll look into it soon