Solr ignores bool filter attributes that are set as integer

I've observed this behaviour when playing around with configuring synonyms. I recognized that the SynonymFilterFactory didn't expand the search terms although it was configured so. The configuration looked like this:

<filter class="solr.SynonymFilterFactory" synonyms="synonyms_de.txt" expand="1" ignoreCase="1"/>

I tried to replace the integers by real boolean values and changed the line to this:

<filter class="solr.SynonymFilterFactory" synonyms="synonyms_de.txt" expand="true" ignoreCase="true"/>

After that change, the search terms got expanded like expected.

@mkalkbrenner told me in our chat that this behaviour changed a lot of times in the past but should be stable now since Solr 6.4

In my setup I'm using Solr 6.6.5 on a platform.sh hosting.

PS: I've chosen the issue title to be more general because I expect that Solr acts the same way for any filter. The only thing I really tried and played with is the SynonymFilterFactory and the "expand" attribute like described above