[Fix #1415] Handle backslash-concatenation

In StringLiteralsInInterpolation it's not enough to check
if a string is within a :dstr node. It must be inside
a :begin node that's inside a :dstr node. That's what
constitutes an interpolation.

Add spec examples for some cases that were missing.
Merge pull request #1417 from jonas054/1415_StringLiteralsInInterpolation_bug

[Fix #1415] Handle backslash-concatenation