Add a default gradient fill type (#1042)

Fixes #1032
diff --git a/lottie/src/main/java/com/airbnb/lottie/parser/GradientFillParser.java b/lottie/src/main/java/com/airbnb/lottie/parser/GradientFillParser.java
index 9dadadd..b99ca1b 100644
--- a/lottie/src/main/java/com/airbnb/lottie/parser/GradientFillParser.java
+++ b/lottie/src/main/java/com/airbnb/lottie/parser/GradientFillParser.java
@@ -24,7 +24,7 @@
     GradientType gradientType = null;
     AnimatablePointValue startPoint = null;
     AnimatablePointValue endPoint = null;
-    Path.FillType fillType = null;
+    Path.FillType fillType = Path.FillType.WINDING;
     boolean hidden = false;
 
     while (reader.hasNext()) {