Allow passing Symbols as aliases in Arel

Table names, column names, and table aliases all allow symbols. This brings back symbol support for `#as` on nodes for consistency with these other methods.

Author: benedikt

activerecord/lib/arel/alias_predication.rb

@@ -3,6 +3,8 @@
 module Arel # :nodoc: all
   module AliasPredication
     def as(other)
+      other = other.name if other.is_a?(Symbol)
+
       Nodes::As.new self, Nodes::SqlLiteral.new(other, retryable: true)
     end
   end

activerecord/test/cases/arel/nodes/as_test.rb

@@ -18,6 +18,12 @@ module Nodes
           as = attr.as("foo")
           assert_kind_of Arel::Nodes::SqlLiteral, as.right
         end
+
+        it "converts right to SqlLiteral if a symbol" do
+          attr = Table.new(:users)[:id]
+          as = attr.as(:foo)
+          assert_kind_of Arel::Nodes::SqlLiteral, as.right
+        end
       end
 
       describe "equality" do