Define a function named word_count that counts the number of times words occur in a given string. The function must accept a string as a parameter and return a dictionary in which the key is every unique word and its value is the number of times that word occurs in the string. For example, passing in the string "Rock ties with rock" should return {'rock': 2, 'ties': 1, 'with': 1 }. Categorization should not be case-sensitive. A string with no words should return an empty dictionary.