input: Write a python function to filter a DataFrame for rows with a specific author using pandas.
  assert df_filtered_equals(df, "Rowling").equals(df[df['author'].str.contains("Rowling")])
  assert df_filtered_equals(df, "Tolkien").equals(df[df['author'].str.contains("Tolkien")])
  assert df_filtered_equals(df, "Martin").equals(df[df['author'].str.contains("Martin")])
output: def df_filtered_equals(df, author):
      return df[df['author'].str.contains(author)]