Passing unlisted chart types as-is to Chart.js

I noticed a bug/missing feature for when passing values to chart-type that aren't listed in the switch statement of cleanChartName. The current implementation tries to reference a chart function with an empty string if the type isn't found. I found this restrictive since I wouldn't be able to use the directive to create custom charts introduced by extending Chart.js with plugins. 

I fixed this by letting an unknown type pass through the switch statement in cleanChartName. I also added a throw statement for when the type is missing entirely. So now when using the directive like this:

```html
<canvas tc-chartjs chart-type="MySpecialLineChart" chart-data="data" chart-options="options" width="300" height="300"></canvas>
```

the directive will call the MySpecialLineChart function of Chart.js, provided it's been defined. 

In addition, I found the tests not to cover these cases of referencing the Chart.js instance for the type-specific functions, so I improved them to catch the case of passing unlisted types unmodified. 

What do you think?
would be good to merge this change, to catch no chart type given.