input: Write a function to fetch the source code of a website and save it to a file.
  assert fetch_and_save_source("http://example.com", "example_source.txt") == "example_source.txt"
  assert fetch_and_save_source("http://test.com", "test_source.txt") == "test_source.txt"
  assert fetch_and_save_source("http://example.org", "example_org_source.txt") == "example_org_source.txt"
output: import requests
  
  def fetch_and_save_source(url, filename):
      response = requests.get(url)
      with open(filename, 'w', encoding='utf-8') as file:
          file.write(response.text)
      return filename