Add replace to build proper encoded URL

In the case of a font with spaces in the name (e.g. Roboto Slab), the import url will not be valid and fonts will not load as expected.

src/themes/default/globals/site.variables

@@ -12,7 +12,7 @@
 @headerFont        : @fontName, 'Helvetica Neue', Arial, Helvetica, sans-serif;
 @pageFont          : @fontName, 'Helvetica Neue', Arial, Helvetica, sans-serif;
 
-@googleFontName    : @fontName;
+@googleFontName    : replace(@fontName, ' ', '+');
 @importGoogleFonts : true;
 @googleFontSizes   : '400,700,400italic,700italic';
 @googleSubset      : 'latin';