Added migration script for old cassettes

tests/unit/test_migration.py

@@ -0,0 +1,36 @@
+import filecmp
+import json
+import shutil
+
+import vcr.migration
+
+
+def test_try_migrate_with_json(tmpdir):
+    cassette = tmpdir.join('cassette').strpath
+    shutil.copy('tests/fixtures/migration/old_cassette.json', cassette)
+    assert vcr.migration.try_migrate(cassette)
+    with open('tests/fixtures/migration/new_cassette.json', 'r') as f:
+        expected_json = json.load(f)
+    with open(cassette, 'r') as f:
+        actual_json = json.load(f)
+    assert actual_json == expected_json
+
+
+def test_try_migrate_with_yaml(tmpdir):
+    cassette = tmpdir.join('cassette').strpath
+    shutil.copy('tests/fixtures/migration/old_cassette.yaml', cassette)
+    assert vcr.migration.try_migrate(cassette)
+    assert filecmp.cmp(cassette, 'tests/fixtures/migration/new_cassette.yaml')
+
+
+def test_try_migrate_with_invalid_or_new_cassettes(tmpdir):
+    cassette = tmpdir.join('cassette').strpath
+    files = [
+        'tests/fixtures/migration/not_cassette.txt',
+        'tests/fixtures/migration/new_cassette.yaml',
+        'tests/fixtures/migration/new_cassette.json',
+    ]
+    for file_path in files:
+        shutil.copy(file_path, cassette)
+        assert not vcr.migration.try_migrate(cassette)
+        assert filecmp.cmp(cassette, file_path)  # shold not change file