Parser bug

When forming a CSSDeclaration, the .getProperty is always formed in lowercase.

This error is critical when we are dealing with variables.

--A != --a

And when call .getExpressionAsCSSString, the case is respected, as a result, we lose the relationship

Reproducing an error: https://jsfiddle.net/dauhk27r/

[Panthevm]

Input:

body {
  --A:red;
  background-color:var(--A);
}

Output:

body {
  --a:red;
  background-color:var(--A);
}

var(--A) - variable --A not found

[Panthevm]

There are no other problems. Many thanks!