Content functions incorrectly stringified

[dlwalsh]

We've noticed a regression in our codebase where previously the line:

content: 'counters(section,".") ". "'

correctly produced the following CSS:

content:counters(section,".") ". ";

As of 2.20.28 and up to the current version (2.20.38) it now produces:

content:"counters(section,".") ". "";

There are a few special functions for content that ought not be stringified. Namely attr(), counter(), counters(). Plus maybe others such as calc().