url: call toString before valueOf when stringifying

[TimothyGu]

The ES addition operator calls the ToPrimitive() abstract operation without hint String, leading a subsequent OrdinaryToPrimitive() to call valueOf() first on an object rather than the desired toString().

Instead, use template literals which directly call ToString() abstract operation, per Web IDL spec.

This issue is particularly visible in the example below:

const { URLSearchParams } = url;
const searchParams = new URLSearchParams();
searchParams.set('name', {
  toString() { return 'toString'; },
  valueOf() { return 'valueOf'; }
});
console.log(searchParams.get('name'));

In the browser, the above prints toString, while in current master it prints valueOf.

Fixes: b610a4d "url: enforce valid UTF-8 in WHATWG parser"
Refs: b610a4d#commitcomment-21200056
Refs: https://tc39.github.io/ecma262/#sec-addition-operator-plus-runtime-semantics-evaluation
Refs: https://tc39.github.io/ecma262/#sec-template-literals-runtime-semantics-evaluation

/cc @nodejs/url

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

url

[mscdex]

Second commit message is a tad too long.

[jasnell]

CI: https://ci.nodejs.org/job/node-test-pull-request/6751/

[TimothyGu]

Commit message fixed.

One more CI: https://ci.nodejs.org/job/node-test-pull-request/6797/

[TimothyGu]

Landed in 99b27ce.

[italoacasas]

@TimothyGu this is not landing clearly in v7.x, can backport?

[TimothyGu]

@italoacasas, I'll open a separate PR for many recent URL changes.

[italoacasas]

Thank you @TimothyGu