Edited Unassigned: Created & Modified doc properties cause unreadable content [15206]

Setting the document core properties: 'created' or 'modified' will result in unreadable content.

The following is written:
```
<dcterms:modified">2015-02-18T17:14:22Z</dcterms:modified>
```

However, the following needs to be written instead: (note the "xsi:type="dcterms:W3CDTF" attribute)
```
<dcterms:modified xsi:type="dcterms:W3CDTF">2015-02-18T17:14:22Z</dcterms:modified>
```

This is with the latest source and tested against Office 2013 Excel.

I'm not certain the best way to add an attribute named "xsi:type" to the OfficeProperties.Created and OfficeProperties.Modified setter methods. Attempting to add that attribute after the fact results in the following which still doesn't quite work.

```
<dcterms:modified type="dcterms:W3CDTF">2015-02-18T17:14:22Z</dcterms:modified>
```

Looking at "Sample1" in the demo source, adding code to set the created/modified dates demonstrates this problem.

```
// set some document properties
package.Workbook.Properties.Title = "Invertory";
package.Workbook.Properties.Author = "Jan Källman";
...
package.Workbook.Properties.Modified = DateTime.Now; // Causes unreadable content
package.Workbook.Properties.Created = DateTime.Now; // Causes unreadable content

```