Custom attributes get cleared by SynonymFilter [LUCENE-6667]

I believe the Lucene API enables users to define their custom attributes (by extending `AttributeImpl`) which may be added by custom Tokenizers.
It seems, the `clear` and `copyTo` methods must be implemented to clear and restore the state of this custom attribute.

However, some filters (in our case the SynonymFilter) simply call `AttributeSource.clearAttributes` without invoking `copyTo`. Instead the filter just resets some known attributes, simply ignoring all other custom attributes. In the end our custom attribute value is lost.

Is this a bug in `SynonymFilter` (and others) or are we using the API in the wrong way?

A solution might be of course to provide empty implementations of `clear` and `copyTo`, but I'm not sure if this has other unwanted effects.

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Migrated from [LUCENE-6667](https://issues.apache.org/jira/browse/LUCENE-6667) by Oliver Becker

Start by tracing SynonymFilter's use of AttributeSource.clearAttributes and compare it with the custom AttributeImpl clear and copyTo contract described in the issue. Reproduce the loss of a custom attribute through the filter, then confirm the API behavior and add coverage if the intended behavior is established.

Written by the indexing model from the issue text.