Fixed character region parsing in FontDescription

First off, the default ASCII characters were always included.

Second, the CharacterRegion was ignored due to incorrect deserialization.
Example:
```
      <CharacterRegion>
        <Start>32</Start>
        <End>127</End>
      </CharacterRegion>
```
This was deserialized as "add characters from `3` to `1`", which added nothing.

Thanks to these two mistakes the character region was always 32-126.
Ok...  I think the actual problem starts here:

https://github.com/mono/MonoGame/blob/develop/MonoGame.Framework.Content.Pipeline/Serialization/Intermediate/ElementSerializerT.cs#L39

I noticed during https://github.com/mono/MonoGame/pull/3279 that our XML deserializer seemed to have this extra char array from string thing that XNA didn't do.  This I suspect is the beginning of the issue...  but I expect there is more.

I just noticed the FontDescription deserialization test...

https://github.com/mono/MonoGame/blob/develop/Test/Assets/Xml/19_FontDescription.xml#L18

... doesn't include simple integer regions like that.  I just tried it and it does indeed fail.

If you don't mind me working out a fix here I can do that.