refactor text render

Support G inherit props.
Refactor text render. Text glyphs will perfectly draw along the path

lib/extract/extractStroke.js

@@ -17,16 +17,11 @@ const joins = {
 
 export default function(props) {
     let {stroke} = props;
-    if (!stroke) {
-        return null;
-    }
 
     let strokeWidth = +props.strokeWidth;
 
     if (_.isNil(props.strokeWidth)) {
-        strokeWidth = 1;
-    } else if (!strokeWidth) {
-        return;
+        strokeWidth = null;
     }
 
     let strokeDasharray = props.strokeDasharray;
@@ -40,10 +35,6 @@ export default function(props) {
         strokeDasharray.push(strokeDasharray[0]);
     }
 
-    if (!stroke) {
-        stroke = '#000';
-    }
-
     return {
         stroke: extractBrush(stroke),
         strokeOpacity: extractOpacity(props.strokeOpacity),