Allow picture to not have a 'tag' attribute and add tests for responsive_image module.

Problem/Motivation

In issue #2009662: [REGRESSION] Replace theme() with drupal_render() in picture module a fix was committed in the comment #30 #2009662-30: [REGRESSION] Replace theme() with drupal_render() in picture module and was reopened to add the tests.

Remaining tasks

• Test the alt tag Do we care that the alt tag shows up in the markup?
• Fix the tests.
• Fix the bug if there is one.

Proposed resolution

Not sure this resolution tex is relevant anymore as the code has changed. Old text:
We are opening this new issue for that specific task.
These people contributed on the creation of the original patches: Eric_A, jessebeach, pwieck, porchlight, jeanfei, InternetDevels, lokapujya
And should be mentioned when committing the fix for this issue.