ensureSlash: Fix accidental string-to-NaN coercion

Created by: wchargin

Summary: The hasSlash method uses path.substr(path, path.length - 1) to remove the last character from path. Clearly, the first parameter is suspect; it should be 0. The code works as written, but only very accidentally: the first parameter is coerced by ToNumber to NaN, which is then coerced by ToInteger to +0, per the spec.

Test Plan: Reading the spec should be sufficient. To verify in the Real World:

const path = "has-slash-but-does-not-need-slash/"
const a = path.substr(path, path.length - 1);
const b = path.substr(0, path.length - 1);
console.log(a === b);  // true
console.log(a);        // has-slash-but-does-not-need-slash

wchargin-branch: ensureslash-accidental-coercion