Allow Subclasses of backend models to be indexed

[superdav42]

Some sites may have attributes with custom backend models. We should allow them to be indexed provided they extend one of the backend models we support.

[afoucret]

Hi @superdav42,

If you were able to fix the build, I would be able to merge this one in the master branch.

Build error detail :

FILE: .../Model/Eav/Indexer/Fulltext/Datasource/AbstractAttributeData.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 169 | ERROR | Missing blank line before return statement
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Time: 28.16 secs; Memory: 58Mb

It is mostly a code style problem.

[superdav42]

Build fixed