input: Write a function in Python to create a random string of 10 characters. The function should take one argument, which is the length of the string to be generated. If no length is provided, the function should default to generating a string of 10 characters. The function should return the generated random string.
  
  ### Function Header:
      def generate_string(string_length=10):
  
  ### Assert Test Cases:
      assert len(generate_string())==10
      assert len(generate_string(5))==5
      assert len(generate_string(20))==20
      assert len(generate_string(0))==0
      assert len(generate_string(1))==1
output: import random
  import string
  
  def generate_string(string_length=10):
      return ''.join(random.choices(string.ascii_letters, k=string_length))