Fix unescaped whitespace in is_whitespace/1

[nickdichev]

Hi, first of all thanks for the library.

I think version 0.6.0 of :unicode_set broke this library, I was getting a compilation error:

== Compilation error in file lib/unicode_guards.ex ==
** (Unicode.Set.ParseError) Unable to parse "[[\t-\r][:Zs:]]". expected range while processing sequence inside set inside property or empty set or set. Detected at "[\t-\r][:Zs:]]".
    lib/unicode_set.ex:72: Unicode.Set.parse!/1
    expanding macro: Unicode.Set.match?/2
    lib/unicode_guards.ex:72: Unicode.Guards.is_whitespace/1
    (elixir 1.10.1) expanding macro: Kernel.and/2
    lib/unicode_guards.ex:72: Unicode.Guards.is_whitespace/1
    (elixir 1.10.1) expanding macro: Kernel.Utils.defguard/2
    lib/unicode_guards.ex: Unicode.Guards.is_whitespace/1

I noticed that :unicode_set is pinned at version 0.5.1 but I got the latest 0.7.0 when I pulled the dep.

I think this should fix the issue. I got the library to compile and the tests are passing!

[kipcole9]

Really appreciate the PR and sorry for the sloppy release testing on my side. All fixed now and published on hex as unicode_guards version 0.3.1. The changelog is:

Bug Fixes

• Fix escape sequences in is_whitespace/2 guard. Thanks to @nickdichev, closes Fix unescaped whitespace in is_whitespace/1 #1