Responsive image srcset fallback with src output

[uncenteredDiv]

Description

Adding srcSet to <Image> Component removes the src Attribute.
This PR will always output the src Attribute, even when a srcSet Object is passed.

This will improve the support for older Browsers, since they might don't support srcSet (like IE11). Using the Progressive Enhancement Approach newer Browsers can benefit from the srcSet and will ignore the src Attribute.

Motivation

Improve Support for older Browsers without the need for a srcSet Polyfill - which is difficult to implement when using Jss or Router Links.

How Has This Been Tested?

Tests for Image updated to test for srcSet / sizes Attributes output.
I used the same values as in the Styleguide Image Example.
Running npm run test inside root or /packages/sitecore-jss-react will run the new test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the Contributing guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.

[sc-illiakovalenko]

@unterweltler Thank you for contributing! It will solve problem in IE11.

[petervangennip]

This would be a nice addition for the jss-vue packages too.