Handle Enumerable values on Repository#delete

A test for `RDF::Enumerable` inputs was added by
ruby-rdf/rdf-spec#39. This adds conformance, and
allows `SPARQL::Client::Repository` to use the new
`Mutable#delete_insert` interface. It does not yet implement an
effecient SPARQL `#delete_insert`.

It may be possible to refactor `#delete_statements` in response to these
changes, to remove the code smells called out in the comment in that
method.

lib/sparql/client/repository.rb

@@ -192,10 +192,27 @@ def clear_statements
  # considered to be a pattern, and used to query
  # self to find matching statements to delete.
  #
- # @param [Enumerable<RDF::Statement>] statements
- # @raise [TypeError] if `self` is immutable
- # @return [Mutable]
+ # @overload delete(*statements)
+ # @param [Array<RDF::Statement>] statements
+ # @raise [TypeError] if `self` is immutable
+ # @return [self]
+ #
+ # @overload delete(statements)
+ # @param [Enumerable<RDF::Statement>] statements
+ # @raise [TypeError] if `self` is immutable
+ # @return [self]
+ #
+ # @see RDF::Mutable#delete
  def delete(*statements)
+ statements.map! do |value|
+ if value.respond_to?(:each_statement)
+ delete_statements(value)
+ nil
+ else
+ value
+ end
+ end
+ statements.compact!
  delete_statements(statements) unless statements.empty?
  return self
  end
@@ -263,7 +280,6 @@ def query_pattern(pattern, options = {}, &block)
  # @param [RDF::Enumerable] statements
  # @return [void]
  def delete_statements(statements)
-
  constant = statements.all? do |value|
  # needs to be flattened... urgh
  !value.respond_to?(:each_statement) && begin