Add string_swap.rb file.
Add swap method on string.
String#swap takes two arguments.
Pass "it should replace an occurence of one substring with another" and "it should replace all occurences of one substring with another" tests using gsub.
Pass "it should replace an occurence of one substring with another and vice versa" and "it should replace an occurence of one substring with another and vice versa" using temporary gsub.
Add pending test to make sure it works without temporarily replacing one of the substrings with a substring that already exists in the string.
Fix "should reject arguments that are not a string or array of strings" test.
Pass "it should reject arguments that are not a string or array of strings" test.
Pass "it should allow arrays of strings as arguments" test.