Add support for string source in EndOfLine cop

`ProcessedSource` supports string sources, but `EndOfLine` cop would fail. This
ensures that `EndOfLine` cop supports strings.

* expose original source of `ProcessedSource`
* simplify `EndOfLine` cop logic
Merge pull request #1203 from gylaz/end-of-line-string-support

Add support for string source in EndOfLine cop