Fix for #1451

Line codec was using an undefined variable which caused it to throw an exception.

Fixed and added tests for line codecs

Closes #1451
we should use 'let' here

```
let (:event) { LogStash::Event.new.... }
```

Otherwise weird stuff happens.

`let` is a way for rspec to let us say "These values should be available for all tests in this section" but it gets you a new instance of the value for each test run so that you can make changes in one test, if needed, but those changes are lost (yay!) between tests so you don't mess things up.