Fixing alternatives with higher priority for converters

Closes #876.

At this time this code works fine when the application wants to override a bundled converter:

```java
public MyLongConverter extends LongConverter {}
```

But this not work:

```java
public MyLongConverter implements Converter<Long> {}
```

So this code fixes this behavior, overriding the internal converter list when a new converter was found for the same type.
I guess you can remove the cache field then.