Add missing string sanitation

[bencon]

This PR fixes issue Missing string sanitation before passing failure text to xmlbuilder #27

Below is an example of TAP input that would causes tap-xunit to throw an exception and abort. Adding the missing string sanitation to strip out an odd unicode character fixes this.

TAP version 13
not ok 99 - some test
  ---
    name: AssertionError
    message: " Uncaught TypeError: Cannot read property 'slice' of undefined\\n\\n %c\U0001F477‍ This is a development only message. It will be removed in production builds.\""
    assertion: fail
    at: 'forEach (someFile)'
  ...
# blah blah

1..105
# tests 105
# pass 100
# skip 4
# fail 1

[bencon]

@aghassemi Mind taking a quick peek at this one liner fix? Thanks!