`get-unit` returns wrong value type (causing unintended behavior in certain cases).

In Less 2.6.1 the genCSS method of Anonymous tree nodes does not take care to properly stringify its value when sent to output.add.

When source-maps are enabled, this results in an error SyntaxError: chunk.split is not a function because SourceMapOutput always expects its first chunk parameter to be a string on which it can call the split method.

From a cursory glance at the codebase, it seems Less 2.7.x still has this problem as well.

A minimal reproducing case:

.test { value : get-unit(10px); }

Proposed fix:

Anonymous.prototype.genCSS = function (context, output) {
    var strValue = this.value != null ? String(this.value) : "";
    output.add(strValue, this.currentFileInfo, this.index, this.mapLines);
};

(Sorry guys; I don't have time to set up a full pull request. I hope someone can take this and do the legwork.)

Locate Anonymous.prototype.genCSS and the SourceMapOutput path mentioned in the issue. Reproduce the get-unit(10px) case with source maps enabled, then verify that the generated value is passed as a string and the chunk.split error no longer occurs.