fix angular-pdf: add default for canvasid

- [x] add default to canvasid var
- [x] delete not used if statament

the default fix the statament on [line 43](https://github.com/sayanee/angularjs-pdf/blob/master/dist/angular-pdf.js#L43)
return true when canvasid is not defined on html.

Author: simobasso

dist/angular-pdf.js

@@ -38,13 +38,8 @@
         var pdfDoc = null;
         var pageNum = (attrs.page ? attrs.page : 1);
         var scale = attrs.scale > 0 ? attrs.scale : 1;
-        var canvas;
-
-        if (attrs) {
-          canvas = document.getElementById(attrs.canvasid);
-        } else {
-          canvas = document.getElementById('pdf-canvas');
-        }
+        var canvasid = attrs.canvasid || 'pdf-canvas';
+        var canvas = document.getElementById(canvasid);
 
         var creds = attrs.usecredentials;
         var ctx = canvas.getContext('2d');

dist/angular-pdf.min.js

@@ -38,13 +38,8 @@
         var pdfDoc = null;
         var pageNum = (attrs.page ? attrs.page : 1);
         var scale = attrs.scale > 0 ? attrs.scale : 1;
-        var canvas;
-
-        if (attrs) {
-          canvas = document.getElementById(attrs.canvasid);
-        } else {
-          canvas = document.getElementById('pdf-canvas');
-        }
+        var canvasid = attrs.canvasid || 'pdf-canvas';
+        var canvas = document.getElementById(canvasid);
 
         var creds = attrs.usecredentials;
         var ctx = canvas.getContext('2d');