fix 4 lint errors

src/core.js

@@ -31,19 +31,19 @@ function getPdf(arg, callback) {
     params = { url: arg };
 
   var xhr = new XMLHttpRequest();
-  
+
   xhr.open('GET', params.url);
-  
+
   var headers = params.headers;
   if (headers) {
     for (var property in headers) {
       if (typeof headers[property] === 'undefined')
         continue;
-      
+
       xhr.setRequestHeader(property, params.headers[property]);
     }
   }
-  
+
   xhr.mozResponseType = xhr.responseType = 'arraybuffer';
   var protocol = params.url.indexOf(':') < 0 ? window.location.protocol :
     params.url.substring(0, params.url.indexOf(':') + 1);