[Ruby 2.5] Add spec for String#start_with? supporting regexp

[nilsding]

This PR adds a spec for Feature #13712: String#start_with? with regexp.

[marcandre]

This is a good start.
I don't think the spec for case sensitivity is needed. We can't test all the possible regexp scenario here, and there's nothing special about case sensitivity and start_with?. Maybe it would be good instead to check what happens if the regexp uses ^ and/or $.
Also, is Regexp.last_match ($1, $2, ...) kept intact or not.
Would you like to amend your PR?

[eregon]

@nilsding Could you improve the PR based on @marcandre's review?

[nilsding]

Oh, of course! Kinda forgot about that, the past few weeks have been a bit stressful for me.

[nilsding]

Do you think it would make sense to add another expectation for something along "chunky\n12bacon" (would return false in this case)

[marcandre]

I guess "chunky\n12bacon".start_with?(/12/).should be_false might be a good idea, just to be sure that it doesn't test for a line starting with, but the whole string starting with

[nilsding]

good idea indeed, I'll add that one

[eregon]

This looks great, thank you for the new spec!