Fix for XML not updating when blank or nil values are sent via JSON using REST API

[codycooperross]

Purpose

closes: datacite/datacite#2264

Approach

The read_attrs method was only used in one place to detect when @params 1) contains no values except xml or 2) contains values that might initiate XML regeneration, then using that condition to set the regenerate parameter. This PR rewrites that method to detect any changes, including blank or nil values, anywhere in the metadata-related values of the @params aside from schemaVersion, which is set earlier in the cleanse method and thus always has a value.

Open Questions and Pre-Merge TODOs

Learning

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)

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