Add Tests for WebAPI::DBIC::Resource::ActiveModel::Role::ItemWritable

[lohengrin332]

WebAPI::DBIC::Resource::ActiveModel::Role::ItemWritable currently has very low test coverage at present. This module needs to be verified to support the correct format and tests created accordingly.

[timbunce]

Interesting discussion here which probably ought to be linked to some somewhere in WAPID. We probably need a "how to choose a media type" doc somewhere.