Cast handling fee to float

[schmengler]

Description

PHP 7.1 complains with a warning if non-numeric strings are used in calculations (see: http://php.net/manual/en/migration71.other-changes.php)

The handling fee configuration of shipping methods is often an empty string. Prior to PHP 7.1 it was silently casted to 0, now this should happen explicitly to avoid warnings.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @okorshenko, thank you for the review.
MAGETWO-87934 has been created to process this Pull Request

[magento-engcom-team]

@schmengler thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.