Only convert attributes on the request that actually exist
merged branch igorw/convert-only-existing-attribute (PR #769)

This PR was squashed before being merged into the master branch (closes #769).

Discussion
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Only convert attributes on the request that actually exist

Fixes #768.

Note: This could be considered a BC break, since it on longer implicitly creates attributes on the request. I'd consider it quite a minor break. The name `convert` implies that the attribute must be present, and I personally would  consider this a bug fix instead.

Commits
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f2fc754 Only convert attributes on the request that actually exist