Change sha1 mutation hash to use a separator between strings

* When concatenating strings together to hash you need to use a
  separator that does not exist in the strings, otherwise you can
  easily get duplicate hashes, eg:

    Digest::SHA1.hexdigest('ab' + 'c')  # => "a9993e364706816aba3e25717850c26c9cd0d89d"
    Digest::SHA1.hexdigest('a' + 'bc')  # => "a9993e364706816aba3e25717850c26c9cd0d89d"

  Using a null character as a separator works around this problem:

    Digest::SHA1.hexdigest('a' + 0.chr + 'bc')  # => "0b2749668f0ea8df8a630da13f0d218709efd5ca"
    Digest::SHA1.hexdigest('ab' + 0.chr + 'c')  # => "dbdd4f85d8a56500aa5c9c8a0d456f96280c92e5"

lib/mutant/mutation.rb

@@ -97,7 +97,7 @@ module Mutant
     # @api private
     #
     def sha1
-      Digest::SHA1.hexdigest(subject.identification + source)
+      Digest::SHA1.hexdigest(subject.identification + 0.chr + source)
     end
     memoize :sha1