Updating URL escaping function.

[emrikol]

esc_url() should be used for src attributes instead of esc_attr()

Summary

This PR can be summarized in the following changelog entry:

Addresses issue #

Relevant technical choices

Checklist:

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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[emrikol]

Closing: Not worth signing the CLA over this 🤷‍♂️

[swissspidy]

No worries @emrikol, we'll try to fix this issue nonetheless.

[ghost]

@googlebot @swissspidy
I think this update should be approved!

[swissspidy]

@sviluppomania if the CLA is not signed by the author we cannot merge pull requests.

However, this definitely encourages us to audit existing escaping functions and to potentially improve the code base in this regard.