Recreation of PR #214 as updating that PR failed.

Props @ccrims0n

Add json() for json responses

• Add json() which returns decoded json from the response body
  if the response body is a valid json else throws an exception

Fix json_decode() for php 5.2 and 5.3

Fix json() for php5.4

Response::json(): revert changes for PHP 5.2 -5.4 compatibility

PHP 5.6 is the new minimum PHP version, so these accommodations for PHP 5.2 - 5.4 can be reverted again.

This reverts commit 8ea4334 and 2d2ecf3.

Response::json(): update to make code runnable again

• Use namespaced names.
• Namespace test class and rename test file to match.
• Use expectException() method instead of annotation.

Response::json(): update to comply with coding standards

Response::json(): rename the method

... to be more descriptive.

Response::decode_body(): rename parameter

... to be in-line with the parameter name used in PHP itself.

Ref: https://www.php.net/json-decode

Response::decode_body(): modernize the error handling

As of PHP 5.5.0, PHP contains the json_last_error_msg() function which will return the last error message.

This removes the need for the error decoding array, which was outdated by now anyway due to new error codes having been introduced in PHP since the PR was originally pulled.

Ref: https://www.php.net/manual/en/function.json-last-error-msg.php

Response::decode_body(): improve the method documentation

... to be in-line with the documentation for the same parameters in the PHP native function.

Ref: https://www.php.net/json-decode

Response::decode_body(): improve the tests

• Use type safe assertSame() instead of assertEquals() and remove the unnecessary foreach.
• Rework the "invalid JSON" test to a data provider and add some adidtional test cases.
• Add @covers tags.
• Add @requires tags for the JSON extension as while that extension has been bundled with PHP for a long time, it could still be disabled until PHP 8.0.
• Add documentation.

Composer: make the dependency on the JSON extension explicit

Fixes #167
Closes #214