Ensure correct serialization of searchSynonyms' type query parameter

[aseure]

When performing a index.searchSynonyms() call, a type query
parameter can
be passed to restrict the type(s) of synonyms one wants to search for.

@maxiloc reported us that the Java implementation is accepting an array of
strings for this parameter. However, the engine only accepts a comma-separated
string for this. We then need to double check the implementation of the
serialization of this parameter and provide a fix if needed.

[nunomaduro]

This is a non-typed API Client. Therefore, nothing to perform here.