Add support for comma separated srcset values in sef.php

System - SEF does not support comma separated values when processing the srcset attribute so only the first URL is converted.

This PR removes srcset from the $attributes array on line 120 and processes instances of the attribute separately, splitting the value by a comma and processing each part individually, then returning the value.

For example,

```html
<img src="images/responsive/winter.jpg" srcset="images/responsive/winter-hd.jpg 2x,images/responsive/winter-2x.jpg 1.5x,images/responsive/winter.jpg 1x" alt="winter" />
```
becomes
```html
<img src="/images/responsive/winter.jpg" srcset="/images/responsive/winter-hd.jpg 2x,/images/responsive/winter-2x.jpg 1.5x,/images/responsive/winter.jpg 1x" alt="winter" /> 
```
whereas before, only the first url in the srcset attribute would have been converted, ie:
```html
<img src="/images/responsive/winter.jpg" srcset="/images/responsive/winter-hd.jpg 2x,images/responsive/winter-2x.jpg 1.5x,images/responsive/winter.jpg 1x" alt="winter" />
```
A more elegant solution may be available, but this one appears to work OK.

### Testing Instructions

Create a new article using the following HTML code, or similar. You will need 3 images of different resolutions, or just three different images of any resolution.
 
```html
<img src="images/image1.jpg" srcset="images/image3.jpg 2x,images/image2.jpg 1.5x,images/image1.jpg 1x" alt="" />
```

### Expected result
The image displays correctly in the browser.

### Actual result
With the current sef.php the images will not display correctly in a modern browser. You can check the code using the browser console to see that the srcset values have not all been converted.

plugins/system/sef/sef.php

@@ -117,16 +117,30 @@ public function onAfterRender()
 
 		// Check for all unknown protocals (a protocol must contain at least one alpahnumeric character followed by a ":").
 		$protocols  = '[a-zA-Z0-9\-]+:';
-		$attributes = array('href=', 'src=', 'srcset=', 'poster=');
-
-		foreach ($attributes as $attribute)
+		$attributes = array('href=', 'src=', 'poster=');
+        
+		foreach ($attributes as $attribute) 
 		{
-			if (strpos($buffer, $attribute) !== false)
+		    if (strpos($buffer, $attribute) !== false) 
+		    {
+			$regex = '#\s+' . $attribute . '"(?!/|' . $protocols . '|\#|\')([^"]*)"#m';
+			$buffer = preg_replace($regex, ' ' . $attribute . '"' . $base . '$1"', $buffer);
+			$this->checkBuffer($buffer);
+		    }
+		}
+
+		if (strpos($buffer, 'srcset=') !== false) 
+		{
+		    $regex = '#\s+srcset="([^"]+)"#m';
+		    $buffer = preg_replace_callback($regex, function ($match) use ($base, $protocols) {
+			$data = array();
+			foreach (explode(",", $match[1]) as $url) 
 			{
-				$regex  = '#\s' . $attribute . '"(?!/|' . $protocols . '|\#|\')([^"]*)"#m';
-				$buffer = preg_replace($regex, ' ' . $attribute . '"' . $base . '$1"', $buffer);
-				$this->checkBuffer($buffer);
+			    $data[] = preg_replace('#(?!/|' . $protocols . '|\#|\')([^\s]+)\s+(.*)#', $base . '$1 $2', $url);
 			}
+			return ' srcset="' . implode(",", $data) . '"';
+		    }, $buffer);
+		    $this->checkBuffer($buffer);
 		}
 
 		// Replace all unknown protocals in javascript window open events.