Add support for IO.readlines(name, options) with implicit separator

This allows a call such as IO.readlines("foo", :encoding => "utf-8"),
instead of raising a Coercion error as before.
Add spec for IO.readlines with implicit separator and options Hash
Merge pull request #2271 from leocassarani/readlines-encoding

Add support for IO.readlines(name, options) with implicit separator