Improper string concatenation w/string functions

[davidkpiano]

TEST:

@function capitalize($string) {
    $char: str-slice($string, 1, 1);

    @return to-upper-case($char) + str-slice($string, 2);
}

.test {
  test: capitalize('fred');
}

EXPECTED: (results per Ruby Sass 3.4.14)

.test {
  test: "Fred";
}

ACTUAL: (tested on LibSass 3.2.5)

.test {
  test: "F"red;
}

[mgreter]

Spec test should also include test for to-lower-case!

[xzyfer]

Specs added sass/sass-spec#413