Fix string interpolation

[syohex]

syntax-ppss(parse-partial-sexp) documents says following. Its value is not always character and I suppose it is enough whether its value is non-nil.

  3. non-nil if inside a string.
     (it is the character that will terminate the string,
      or t if the string should be terminated by a generic string delimiter.)

Before

With this patch

This is related to #206.

[tonini]

👍 ❤️ 💛 💙