Bug fix for remove attribute functionality

Without this patch, if SVG containing attributes not in `pdfSvgAttr` is rendered with option *removeInvalid* set to true, an `Unable to get property 'name' of undefined or null reference` error may be encountered. 

This patch fixes this issue. 

## Cause
Code such as the following which deletes items from the attribute collection **while iterating through that same collection:**
````
$.each(node.attributes, function(i, a) {
    if(typeof(a) != 'undefined' && pdfSvgAttr.g.indexOf(a.name.toLowerCase()) == -1) {
	node.removeAttribute(a.name);
    }
});
````

## Example
In the below code, rendering any immediate child of `<svg>` using IE11 (Windows 7 Pro) causes an `Unable to get property 'name' of undefined or null reference` error.

````
<svg id="score" style="overflow: hidden; position: relative;" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="720" version="1.1" height="288">
	<text invalid-attribute-one="true" invalid-attribute-two="true" x="20" y="140" font-family="serif" font-style="italic" fill="black">Some Text</text>
	<rect invalid-attribute-one="true" invalid-attribute-two="true" x="14" y="23" width="150" height="77" fill="lime"
	      stroke="black" stroke-width="1"></rect>
	<circle invalid-attribute-one="true" invalid-attribute-two="true"  cx="220" cy="100" r="50" stroke="black"
		stroke-width="5" fill="red"></circle>
	<ellipse invalid-attribute-one="true" invalid-attribute-two="true"  cx="158" cy="169" rx="75" ry="20"></ellipse>
	<line invalid-attribute-one="true" invalid-attribute-two="true"  x1="0" y1="0" x2="200" y2="200"></line>
	<g invalid-attribute-one="true" invalid-attribute-two="true">
		<circle cx="25" cy="25" r="15"></circle>
		<circle cx="40" cy="25" r="15"></circle>
	</g>
</svg>
```