Test: review all tests to stabilize them more

Currently, there are a lot of tests which follow a pattern along the lines of:

// Do something
$this->assertSame($expected, $result['key']['subkey']->property);

Tests like these are unstable as they may error out on any of the following:

• $result not being an array.
• The 'key' index not existing in $result
• The 'subkey' index not existing in $result['key']
• $result['key']['subkey'] not being an object
• $result['key']['subkey'] not being an object of the expected type (instance of)
• The object in $result['key']['subkey'] not having a property named $property.

Those tests should be stabilized and improved by changing them to something along the lines of:

// Do something
$this->assertIsArray($result);
$this->assertHaskey('key', $result);
$this->assertIsArray($result['key']);
$this->assertHaskey('subkey', $result['key']);
$this->assertIsObject($result['key']['subkey']);
$this->assertInstanceOf($classname, $result['key']['subkey']);
$this->assertHasAttribute('property', $result['key']['subkey']);
$this->assertSame($expected, $result['key']['subkey']->property);

Additionally, all tests which have multiple assertions, should pass the $message parameter to each assertion to help figure out which assertion has failed when a test fails.

Other review tasks (optional, but highly recommended)

• Refactoring tests methods which do the same thing to data providers.
• Enhancing existing data providers to use named data sets and keyed data entries.
• Adding appropriate @covers tags.
• Reviewing code coverage and adding additional tests to raise (both path as well as branch) code coverage.
• Adding test documentation.
• Potentially splitting test classes which do too much into multiple classes based on what is being covered by the tests.

Actions list

• Requests\Tests\ChunkedEncodingTest - PR #581
• Requests\Tests\CookiesTest - PR series, PR #737
  Owner: @jrfnl
  Status: Ready, waiting to be pulled
• Requests\Tests\EncodingTest - PR #595
• Requests\Tests\IdnaEncoderTest - PR #549
• Requests\Tests\IriTest
• Requests\Tests\RequestsTest
• Requests\Tests\SessionTest
  Owner: @jrfnl
  Status: WIP
• Requests\Tests\SslTest - PR #551
• Requests\Tests\Auth\BasicTest - PR #557
• Requests\Tests\Cookie\JarTest - PR #734
• Requests\Tests\Proxy\HttpTest
• Requests\Tests\Response\HeadersTest - PR #555
• Requests\Tests\Transport\BaseTestCase
• Requests\Tests\Transport\CurlTest
• Requests\Tests\Transport\FsockopenTest
• Requests\Tests\Utility\FilteredIteratorTest - PR #550
  Status: basic review done, could use more improvements and additional tests.

Start by reviewing the completed test cleanups in PRs #581, #595, #549, #551, #557, #555, and #550, then inspect the unchecked classes such as Requests\Tests\IriTest, Requests\Tests\RequestsTest, and Requests\Tests\Proxy\HttpTest. Stabilize unsafe assertions, add messages to multiple assertions, and address the listed optional review tasks; done means the remaining test classes have been reviewed and the suite passes.