(PUP-716) File19Windows#symlink calls exist? on instance not class.

Fixes a bug that was calling exist? on the File19Windows impl class when
it was now an instance method of the same.
Merge pull request #2242 from jpartlow/issue/master/pup-716-fix-exist-call-in-file19windows-impl

(PUP-716) File19Windows#symlink calls exist? on instance not class.