Fix a TypeError of url.parse

[StoneDot]

This PR fixes TypeError occurring when http2.connect is called with url.URL.
It is permitted according to official document.

A following code reproduces the error.

Source:

const tracing = require('@opencensus/nodejs');
const zipkin = require('@opencensus/exporter-zipkin');

const tracer = tracing.start({samplingRate: 1.0}).tracer;

const http2 = require('http2');

tracer.registerSpanEventListener(new zipkin.ZipkinTraceExporter({
    url: 'http://localhost:9411/api/v2/spans',
    serviceName: 'node.js-open-census-bug'
}));

const url = new URL('/index.html', 'https://example.com/');
const client = http2.connect(url);

const {
  HTTP2_HEADER_STATUS
} = http2.constants;
const headers = {};
const req = client.request(headers);
req.on('response', headers => {
  console.log(headers[HTTP2_HEADER_STATUS]);
  req.on('data', chunk => console.log(chunk));
  req.on('end', () => process.exit());
});

Result:

internal/validators.js:125
    throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "url" argument must be of type string. Received type object
    at validateString (internal/validators.js:125:11)
    at Url.parse (url.js:151:3)
    at Object.urlParse [as parse] (url.js:146:13)
    at ClientHttp2Stream.request.on (/Users/h-goto/projects/opencensus-bug-report/node_modules/@opencensus/instrumentation-http2/build/src/http2.js:107:34)
    at ClientHttp2Stream.contextWrapper (/Users/h-goto/projects/opencensus-bug-report/node_modules/@opencensus/core/build/src/internal/cls-ah.js:78:28)
    at ClientHttp2Stream.emit (events.js:202:15)
    at endReadableNT (_stream_readable.js:1132:12)
    at processTicksAndRejections (internal/process/next_tick.js:76:17)

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[draffensperger]

Thanks @StoneDot for the fix! Would you be willing to add a unit test for this?

[mayurkale22]

+1, also update the CHANGELOG.md

[StoneDot]

I've written the tests for this PR, but it hasn't worked correctly because of another bug (See #641).

Could I tweak the tests to pass?

[StoneDot]

Also I've updated CHANGELOG.md at #de29571

[StoneDot]

Currently I'm preparing to contract CLA as a company (Because this contribution was created on work time). It will take more time to sign a CLA. I am sorry for being late.

[googlebot]

CLAs look good, thanks!

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[StoneDot]

Rebased to include #650

[StoneDot]

Could you wait merge? (It seems Googlebot doesn't work correctly for me.)
I have not agreed CLA as a company yet.

[mayurkale22]

@StoneDot Let us know if you are waiting for the next release (0. 0.18) with this fix.

[StoneDot]

@mayurkale22 Thanks! But I'm not in a rush.
However, I'd like to get the new release if #647 is merged (sorry for my slow work).