Scripts without newline cause NoMethodError

When calling something like:

```
assert_script "$ cmd"
```

The result is a NoMethodError.

```
NoMethodError: undefined method `sub!' for nil:NilClass
    /Users/chiangs/.rvm/gems/ruby-1.9.2-p290@linecook/gems/shell_test-0.3.0/lib/shell_test/shell_methods/session.rb:111:in `split'
    ...
```
