Fix for String.unindent()

I noticed that calling unindent() doesn't do the right thing:

irb(main):001:0> require 'facets/string/unindent'
=> true
irb(main):002:0> "  two\n    four\n".unindent
=> "two\n four\n"

Note that there is only one space before 'four'; there should be two.  With this change, I get the correct output.

I ran the whole test suite and got no failures.

Incidentally, figuring out how to run the tests took a while.  It would be nice if Facets include a README-testing.txt file explaining that the tests use a framework called 'Lemon', and where to find it.  Google helped me figure that out.  The documentation on how to run Lemon tests was also out of date - this page:

https://github.com/rubyworks/lemon/wiki/Running-Tests

...advises running the 'lemon' command, when the real command appears to be called 'lemonade'.  And 'lemonade' didn't work for me anyway, producing a LoadError: "no such file to load -- test/cli".  I had to run "rubytest -r lemon test" (the docs say "ruby-test" but that was also incorrect).  I installed using 'sudo gem install lemon' and got lemon 0.9.0 and rubytest 0.3.1.  Let me know if something's wrong with my setup.