Pass ServiceBuilder construct config into children

[jdpedrie]

So I've been investigating the report in #766, and found that the constructor $config is passed to services ONLY when no child $config is set.

For instance:

use Google\Cloud\ServiceBuilder;

putenv('GOOGLE_APPLICATION_CREDENTIALS=/path/to/keyfile1.json');

$cloud = new ServiceBuilder([
    'keyFilePath' => '/path/to/keyfile2.json'
]);

// uses `keyfile2.json`
$datastore = $cloud->datastore();

// uses `keyfile1.json`
$datastore = $cloud->datastore([
    'namespaceId' => 'myApp'
]);

This strikes me as incorrect, as it prevents e.g. SpeechClient from setting a languageCode and using ServiceBuilder-level credentials. My proposed solution simply merges ServiceBuilder and factory method configuration, preferring keys given later.

If accepted, this PR closes #766.

edit: for a demonstration, pull the PR and run vendor/bin/phpunit --group=servicebuilder. Then reset the changes to src/Core/ServiceBuilder.php to the current master branch. Undoing changes to ServiceBuilder will expose errors on SpeechClient.