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.
On the topic of Function names, the standard I follow is to use ```Test_{Target Class}``` as the name of the Test class, and ```{actual method}``` as the name of the test method. See https://github.com/o2platform/FluentSharp/blob/master/UnitTests/UnitTests.FluentSharp.CoreLib/ExtensionMethods/Test_Char.cs for an example of that in action.

In this case this test should be called

```csharp
[Test] public void trim()
```