FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
I realize this is duplicating the existing test above, but maybe that's more of an argument that we could be more clear here what the result is supposed to be here? It took me a second to be sure it was right, and I could see someone flipping the bit if they caused a regression and quickly tried to fix it by changing the test.

```
it('returns that it added one line') { should be == 1 } 
```