Fix for #1760

[seven-phases-max]

Fixes property interpolation (#1760). Now variables of any type (and their combinations) should expand properly.

[lukeapage]

would it be better to give an error than to allow this?

[seven-phases-max]

would it be better to give an error than to allow this?

That's no so simple. Actually I was already about giving up a complete evaluation of a variable in a property when I saw it grows more complicated (and generate some error for "complex expansions" instead) but... But with a error detection it tends to become even more cryptic, consider the following code:

@p: ~' ' ~`1 + 1, "  "` /* foo */ bar;

z {
    @{p}: 0;
}

It's a bit exaggerated example of course but there's no way for the compiler to 100% decide if the property name is correct or it isn't unless we completely re-parse the result string all over again. So my main goal for this fix was just to make sure a variable is expanded "as is", i.e. the same way it works with selector interpolation (or within backticks interpolation), i.e. as in (#1760):

In general LESS never tests if its output is a valid CSS

This way we don't have to invent any new special rules for each interpolation type (and a user at least knows what to expect there w/o any special documentation). And this scary property test above is only to ensure the expansion works properly.