str-slice and str-insert should normalise quote to double quotes
str-slice with $end-at: 0 should never return a value
Merge pull request #762 from xzyfer/fix/str-slice-end-at-0

`str-slice` with `$end-at: `0 should never return a value
Merge pull request #764 from xzyfer/fix/normalise-str-function-quotes

str-slice and str-insert should normalise quote to double quotes