Support SQLAlchemy default in-memory SQLite URL

SQLAlchemy's documentation states if you want to create an in-memory
database you can provide a URL of `sqlite://` and that `:memory` is
implied. The `database_exists` and `create_database` function did not
handle this use-case.

conftest.py

@@ -67,6 +67,11 @@ def sqlite_memory_dsn():
     return 'sqlite:///:memory:'
 
 
+@pytest.fixture
+def sqlite_none_database_dsn():
+    return 'sqlite://'
+
+
 @pytest.fixture
 def sqlite_file_dsn():
     return 'sqlite:///{0}.db'.format(db_name)

sqlalchemy_utils/functions/database.py

@@ -466,7 +466,12 @@ def database_exists(url):
         return bool(engine.execute(text).scalar())
 
     elif engine.dialect.name == 'sqlite':
+        if database:
             return database == ':memory:' or os.path.exists(database)
+        else:
+            # The default SQLAlchemy database is in memory,
+            # and :memory is not required, thus we should support that use-case
+            return True
 
     else:
         text = 'SELECT 1'
@@ -538,6 +543,7 @@ def create_database(url, encoding='utf8', template=None):
         engine.execute(text)
 
     elif engine.dialect.name == 'sqlite' and database != ':memory:':
+        if database:
             open(database, 'w').close()
 
     else:

tests/functions/test_database.py

@@ -27,6 +27,12 @@ class TestDatabaseSQLiteMemory(object):
         assert database_exists(dsn)
 
 
+@pytest.mark.usefixture('sqlite_none_database_dsn')
+class TestDatabaseSQLiteMemoryNoDatabaseString(object):
+    def test_exists_memory_none_database(self, sqlite_none_database_dsn):
+        assert database_exists(sqlite_none_database_dsn)
+
+
 @pytest.mark.usefixtures('sqlite_file_dsn')
 class TestDatabaseSQLiteFile(DatabaseTest):
     pass