Accept Encoding object for Encoding.compatible?

With this change, the following calls are supported:

    Encoding.compatible?(Encoding.find("UTF-8"),
                         "String".force_encoding("UTF-8"))
    # => #<Encoding:UTF-8>

    Encoding.compatible?("String".force_encoding("UTF-8"),
                         Encoding.find("US-ASCII"))
    # => #<Encoding:UTF-8>

    Encoding.compatible?(Encoding.find("UTF-8"),
                         Encoding.find("US-ASCII"))
    # => #<Encoding:UTF-8>
Merge pull request #457 from kou/support-encoding-object-for-encoding-compatible-p

Accept Encoding object for Encoding.compatible?