Bugfix for FSSM::Tree::Cache on UNC.

Cause of a problem:

    Pathname("//host/share").join("hoge.txt")
    # => #<Pathname://host/sharehoge.txt>
    Pathname("//host/share/").join("hoge.txt")
    # => #<Pathname://host/share/hoge.txt>

This makes corruption at FSSM::Pathname.for().join() in FSSM::Tree::NodeEnumerable.each().
Therefore FSSM::Tree::Cache.files is not work correctly on UNC.
This patch modifies the behavior as below.

before:

    FSSM::Pathname.for("//host/share/hoge.txt").segments
    # => ["//host/share", "hoge.txt"]
    cache = FSSM::Tree::Cache.new
    # => #<FSSM::Tree::Cache:0x00000600440b28 @children={}>
    cache.set "//host/share/hoge.txt"
    # => 2014-02-25 11:38:19 +0900
    cache.files
    # => {"//host/sharehoge.txt"=>2014-02-25 11:38:19 +0900}

after:

    FSSM::Pathname.for("//host/share/hoge.txt").segments
    # => ["//host/share/", "hoge.txt"]
    cache = FSSM::Tree::Cache.new
    # => #<FSSM::Tree::Cache:0x0000060003e660 @children={}>
    cache.set "//host/share/hoge.txt"
    # => 2014-02-25 11:38:19 +0900
    cache.files
    # => {"//host/share/hoge.txt"=>2014-02-25 11:38:19 +0900}