Do not force journal mode unless specified (#341)

Let SQLite use the default journal mode if none is given. This is
important because the database may be in WAL mode already (which is
persistent) and forcing it to DELETE mode unexpectedly changes the
database out of WAL mode (also in a persistent way).

Closes #340.

Author: rhcarvalho

lib/exqlite/connection.ex

@@ -464,7 +464,7 @@ defmodule Exqlite.Connection do
   end
 
   defp set_journal_mode(db, options) do
-    maybe_set_pragma(db, "journal_mode", Pragma.journal_mode(options))
+    set_pragma_if_present(db, "journal_mode", Keyword.get(options, :journal_mode))
   end
 
   defp set_temp_store(db, options) do

test/exqlite/connection_test.exs

@@ -171,6 +171,18 @@ defmodule Exqlite.ConnectionTest do
       assert {:error, "attempt to write a readonly database"} ==
                Sqlite3.execute(ro_state.db, insert_value_query)
     end
+
+    test "preserves WAL mode across connections" do
+      path = Temp.path!()
+
+      {:ok, state1} = Connection.connect(database: path, journal_mode: :wal)
+      assert {:ok, "wal"} = get_pragma(state1.db, :journal_mode)
+
+      {:ok, state2} = Connection.connect(database: path)
+      assert {:ok, "wal"} = get_pragma(state2.db, :journal_mode)
+
+      File.rm(path)
+    end
   end
 
   defp get_pragma(db, pragma_name) do