Fix a NPE in RequestBuilderBase.toString()

Author: erwan

api/src/main/java/com/ning/http/client/RequestBuilderBase.java

@@ -294,18 +294,22 @@ public String toString() {
             sb.append("\t");
             sb.append(method);
             sb.append("\theaders:");
-            for (String name : headers.keySet()) {
-                sb.append("\t");
-                sb.append(name);
-                sb.append(":");
-                sb.append(headers.getJoinedValue(name, ", "));
+            if (headers != null) {
+                for (String name : headers.keySet()) {
+                    sb.append("\t");
+                    sb.append(name);
+                    sb.append(":");
+                    sb.append(headers.getJoinedValue(name, ", "));
+                }
             }
             sb.append("\tparams:");
-            for (String name : params.keySet()) {
-                sb.append("\t");
-                sb.append(name);
-                sb.append(":");
-                sb.append(params.getJoinedValue(name, ", "));
+            if (params != null) {
+                for (String name : params.keySet()) {
+                    sb.append("\t");
+                    sb.append(name);
+                    sb.append(":");
+                    sb.append(params.getJoinedValue(name, ", "));
+                }
             }
 
             return sb.toString();