Fixed lower,upper and trim functions. (https://github.com/o2platform/FluentSharp/issues/17)

These commits should fix the issue with upper , lower and trim functions.
this should be 
```csharp
output.lower().assert_Is(output.ToLower())
```

The ```StringComparison.InvariantCulture``` is something that I'm not sure I used on ```assert_Is''' (can you check and if not, send a PR with it)