Fix compile error in D implementation

Relevant output:

    Building all languages ...
      * Entering directory 'languages/D'
        * Running task 'D:clean'... done.
        * Running task 'D:build'... encrypter.d(20): Error: cannot implicitly convert expression (representation(this.message)) of type immutable(ubyte)[] to ubyte[] failed.
          Command failed: 'dmd -w encrypt.d encrypter.d utility.d key_parsing.d':

/cc @reagent