Add default options for guzzle client

[omarlopesino]

Hi.

My use case is this: I need allow unverified https site in tests cause the environment doesn't have certificate.

I tried use key 'defaults' in my behat.yml but I saw this error:

[Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]
Unrecognized option "defaults" under "testwork.web_api"

Here is my behat.yml

default:
autoload: '': %paths.base%/tests/behat/bootstrap
suites:
default:
paths:
- %paths.base%/tests/behat/features
contexts:
- Behat\WebApiExtension\Context\WebApiContext:
extensions:
Behat\WebApiExtension\ServiceContainer\WebApiExtension:
base_url: 'https://rest-testing.me/'
defaults:
verify: false
ssl.certificate_authority: false

I attach a patch adding defaults key.

Can you review this, please? Thanks!