Fix performance issue on large inputs to decodeFlowed()

[valeriansaliou]

Fix for issue reported here: nodemailer/mailparser#389

On my test EML, it produces the same exact same output, in only 11ms vs 9098ms with the previous decodeFlowed().

Once this PR is accepted, the following dependency chain will need to be updated to the fixed libmime version:

- mailparser
  |- mailsplit
    |- libmime

Tests are all passing on my end following this patch.