[SPARK-7782] fixed sort arrow issue

Current behaviour::
In spark UI
![screen shot 2015-05-27 at 3 27 51 pm](https://cloud.githubusercontent.com/assets/3919211/7837541/47d330ba-04a5-11e5-89d1-e5b11da1a513.png)

In YARN
![screen shot 2015-05-27 at 3](https://cloud.githubusercontent.com/assets/3919211/7837594/aebd1d36-04a5-11e5-8216-86e03c07d2bd.png)

In jira
![screen shot 2015-05-27 at 3_2](https://cloud.githubusercontent.com/assets/3919211/7837616/d3fedce2-04a5-11e5-9e68-960ed54e5d83.png)

Author: zuxqoj <[email protected]>

Closes apache#6437 from zuxqoj/SPARK-7782_PR and squashes the following commits:

cd068b9 [zuxqoj] [SPARK-7782] fixed sort arrow issue

core/src/main/resources/org/apache/spark/ui/static/sorttable.js

@@ -100,7 +100,7 @@ sorttable = {
             this.removeChild(document.getElementById('sorttable_sortfwdind'));
             sortrevind = document.createElement('span');
             sortrevind.id = "sorttable_sortrevind";
-            sortrevind.innerHTML = stIsIE ? '&nbsp<font face="webdings">5</font>' : '&nbsp;&#x25B4;';
+            sortrevind.innerHTML = stIsIE ? '&nbsp<font face="webdings">5</font>' : '&nbsp;&#x25BE;';
             this.appendChild(sortrevind);
             return;
           }
@@ -113,7 +113,7 @@ sorttable = {
             this.removeChild(document.getElementById('sorttable_sortrevind'));
             sortfwdind = document.createElement('span');
             sortfwdind.id = "sorttable_sortfwdind";
-            sortfwdind.innerHTML = stIsIE ? '&nbsp<font face="webdings">6</font>' : '&nbsp;&#x25BE;';
+            sortfwdind.innerHTML = stIsIE ? '&nbsp<font face="webdings">6</font>' : '&nbsp;&#x25B4;';
             this.appendChild(sortfwdind);
             return;
           }
@@ -134,7 +134,7 @@ sorttable = {
           this.className += ' sorttable_sorted';
           sortfwdind = document.createElement('span');
           sortfwdind.id = "sorttable_sortfwdind";
-          sortfwdind.innerHTML = stIsIE ? '&nbsp<font face="webdings">6</font>' : '&nbsp;&#x25BE;';
+          sortfwdind.innerHTML = stIsIE ? '&nbsp<font face="webdings">6</font>' : '&nbsp;&#x25B4;';
           this.appendChild(sortfwdind);
 
           // build an array to sort. This is a Schwartzian transform thing,