input: Write a function that uses the KB SRU Python API to perform a search query and retrieves specific information from the search results. The function should take a query string, collection name, start record number, and maximum number of records to return as parameters. The function should return a response object containing the search results, and allow extraction of specific information such as titles, creators, and subjects from the search results.
  
  Here is the function definition and test cases:
  
  
  def sru_search(query, collection='GGC', startrecord=1, maximumrecords=1):
      sru_instance = sru()
      response = sru_instance.search(query, collection=collection, startrecord=startrecord, maximumrecords=maximumrecords)
      return response
  
  # Test cases
  assert isinstance(sru_search("python"), response)
  assert sru_search("python").creators != []
  assert sru_search("data science", collection="DPO").titles != []
  assert sru_search("", collection="BYVANCK").dates != []
  assert sru_search("machine learning", maximumrecords=10).nr_of_records <= 10
  
output: def sru_search(query, collection='GGC', startrecord=1, maximumrecords=1):
      sru_instance = sru()
      response = sru_instance.search(query, collection=collection, startrecord=startrecord, maximumrecords=maximumrecords)
      return response