Add support for tsql 3 and 4 part naming (#279)

Tables may be referenced in SQL Server by up to 4-part naming of
`[server].[database].[schema].[table]`.

These may be passed to the Tds adapter via the schema_prefix as tuples:
`@schema_prefix {"database", "schema"}` and
`@schema_prefix {"server", "database", "schema"}`

lib/ecto/adapters/tds/connection.ex

@@ -1477,8 +1477,14 @@ if Code.ensure_loaded?(Tds) do
 
     defp quote_table(nil, name), do: quote_table(name)
 
+    defp quote_table({server, db, schema}, name),
+      do: [quote_table(server), ".", quote_table(db), ".", quote_table(schema), ".", quote_table(name)]
+
+    defp quote_table({db, schema}, name),
+      do: [quote_table(db), ".", quote_table(schema), ".", quote_table(name)]
+
     defp quote_table(prefix, name),
-      do: Enum.map_join([quote_table(prefix), ".", quote_table(name)], "", &"#{&1}")
+      do: [quote_table(prefix), ".", quote_table(name)]
 
     defp quote_table(name) when is_atom(name), do: quote_table(Atom.to_string(name))
 

test/ecto/adapters/tds_test.exs

@@ -89,6 +89,16 @@ defmodule Ecto.Adapters.TdsTest do
     assert all(query) == ~s{SELECT s0.[binary] FROM [foo].[schema3] AS s0}
   end
 
+  test "from with 2-part prefix" do
+    query = Schema |> select([r], r.x) |> Map.put(:prefix, {"database", "db_schema"}) |> plan()
+    assert all(query) == ~s{SELECT s0.[x] FROM [database].[db_schema].[schema] AS s0}
+  end
+
+  test "from with 3-part prefix" do
+    query = Schema |> select([r], r.x) |> Map.put(:prefix, {"server", "database", "db_schema"}) |> plan()
+    assert all(query) == ~s{SELECT s0.[x] FROM [server].[database].[db_schema].[schema] AS s0}
+  end
+
   test "from without schema" do
     query = "schema" |> select([r], r.x) |> plan()
     assert all(query) == ~s{SELECT s0.[x] FROM [schema] AS s0}