[feaLib] Escape glyph names matching keywords

Glyph names matching reserved keywords were not consistently escaped;
they were escaped in GDEF classes but not elsewhere. Call module’s
asFea() function in GlyohName.asFea() to ensure they are consistently
escaped.

Lib/fontTools/feaLib/ast.py

@@ -160,7 +160,7 @@ class GlyphName(Expression):
         return (self.glyph,)
 
     def asFea(self, indent=""):
-        return str(self.glyph)
+        return asFea(self.glyph)
 
 
 class GlyphClass(Expression):