Fix GCSToGCSOperator copying list of objects without wildcard

[moiseenkov]

Changes:

• fixed GCSToGCSOperator in case copying list of objects without wildcard
• fixed and slightly refactored unit tests
• fixed example DAG

Aforementioned changes of GCSToGCSOperator cover the following cases:

1. Copy a list of files into the folder.

copy_files = GCSToGCSOperator(
    task_id='copy_files_without_wildcard',
    source_bucket=SOURCE_BUCKET,
    source_objects=['src/file_1.txt', 'src/file_2.csv'],
    destination_bucket=TARGET_BUCKET,
    destination_object='new_folder/'
)

The previous implementation didn't actually copy files - it was just creating an empty destination folder. The following fix solves this problem and performs actual copying of the listed files into the specified destination folder.

2. Copy folder without trailing slash

copy_files_from_folder = GCSToGCSOperator(
        task_id='copy_folder_without_trailing_slash',
        source_bucket=SOURCE_BUCKET,
        source_objects=['test_folder'],
        destination_bucket=TARGET_BUCKET,
        destination_object='new_folder/'
    )

For example, we have a folder test_folder/ and a file test_folder/file.txt inside of it. If we miss a trailing slash at the source folder name, then the previous implementation instead of copying the file file.txt were creating two files test_folder and new_folderfile.txt. It seems that there are two bugs here:
a) a file new_folder created instead of a folder new_folder/;
b) a wrong path new_folderfile.txt for the copied file was generated instead of new_folder/file.txt.
The following fix resolves these problems.

[potiuk]

Thanks!