Save background color correctly in images. [backport 2.1]

[raumatbel]

Related with the PR11888

Description

If you updated the background color for image of product, this color always was black and it never was the color indicated in your configuration.

Fixed Issues (if relevant)

1. magento/magento2#8799: Image brackground

Steps to reproduce

1. Edit view.xml file
   ex: theme-frontend-luma/etc/view.xml
2. Add background color attribute to an image

<image id="related_products_list" type="small_image">
     <width>152</width>
     <height>230</height>
     <background>[250,247,247]</background>
</image>

3. Clear cache and reload images.

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)