Make Content-Type Processing Actually Work

Resolves #113 

I really wanted to write up more about what the `[Content-Types].xml` file does but I don't have the NuGet source handy.

Basically we were doing a naive merging of our stuff with their stuff.

So these elements were fine:

``` xml
<Default ContentType="application/octet" Extension="diff"/>
```

 and all the tags like this:

``` xml
<Override ContentType="application/octet" PartName="/Content/frontend/js/library/bootstrap/CNAME"/>
```

Were causing this code to break.