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.
I usually like to have a pile of tests like this (this first could are the ones you had)

```csharp
" "   .trim().assert_Is("").assert_Is(String.Empty)
"    ".trim().assert_Is("").assert_Is(String.Empty)
"  a ".trim().assert_Is("a").assert_Size_Is(1);
" aa ".trim().assert_Is("aa");
"aa  ".trim().assert_Is("aa");
"a a ".trim().assert_Is("a a")
" a a".trim().assert_Is("a a")
(null as string) .trim().assert_Is("").assert_Is(String.Empty)
```