Use `#clear_statements` when available
Use `#clear_statements` when available

`Mutable#clear` checks whether the instance responds to
`#clear_statements` and falls back on `#delete_statements`. The former
clears the entire mutable, using e.g., `graph_name`, while the later has
enumerative semantics (deleting the members of the set of
statements). Because `#clear_statements` is protected or private, it will
never be used by immutable without the `true` flag on `#respond_to?`.

PerfectMemory/rdf,ruby-rdf/rdf