Make Dataset#schema_and_table handle literal strings correctly

Previously, literal strings could be turned into non-literal
strings and thus quoted literal by the identifier quoting code.

lib/sequel/dataset/sql.rb

@@ -654,7 +654,7 @@ def schema_and_table(table_name, sch=(db.default_schema if db))
       when SQL::Identifier
         [sch, table_name.value.to_s]
       when String
-        [sch, table_name.to_s]
+        [sch, table_name]
       else
         raise Error, 'table_name should be a Symbol, SQL::QualifiedIdentifier, SQL::Identifier, or String'
       end

spec/core/dataset_spec.rb

@@ -4465,6 +4465,11 @@ class << Sequel
     @ds.schema_and_table('s').should == [nil, 's']
   end
 
+  it "should correctly handle literal strings" do
+    s = Sequel.lit('s')
+    @ds.schema_and_table(s).last.should equal(s)
+  end
+
   it "should correctly handle identifiers" do
     @ds.schema_and_table(Sequel.identifier(:s)).should == [nil, 's']
   end