Add support for default max tries setting for Http requests

[ugurarici]

WHY are these changes introduced?

Fixes #281

Shopify\Clients\Http is able to retry a request if response code is retriable and tries parameter given to request method. This change gives ability to set a default max tries count for each Http request unless overridden by request method parameter. This way we can also use retries with REST resources.

WHAT is this pull request doing?

Adds a $maxTries parameter to Shopify\Context::initialize method with a default value of 1. Assigns it to Shopify\Context::$MAX_TRIES and uses it as default value in request method of Shopify\Client\Http.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have updated the documentation

[ugurarici]

I have signed the CLA!

[lukeholder]

Fixes #203 and Fixes #281

@paulomarg who is the best person to review things like this?