APIv4 - Fix nonstandard camelCase of title_plural field

[colemanw]

Overview

In a feature recently added by @agh1 and myself, the api returns the name of each entity in plural form as titlePlural. However, api output by convention does not use camelCase. This fixes it, adds the field to the api spec, and backports the test from master with the updated case.

Before

Api returns title and titlePlural for each entity.

After

Api returns title and title_plural for each entity.

Comments

There may be a conflict in forward-merging this to master. I'll fix that when we merge this.

[civibot]

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