Fixed Addressable::URI#inspect to use self.class. (#544)

* This will allow sub-classing `Addressable::URI` and have the
  sub-classes name show up in the `inspect` output.

      class CustomURL < Addressable::URI

        def custom_method
	  ...
	end

      end

      uri = CustomURL.parse("https://example.com")
      # => #<CustomURL:0x1824 URI:https://example.com>

lib/addressable/uri.rb

@@ -2382,7 +2382,7 @@ def to_hash
     #
     # @return [String] The URI object's state, as a <code>String</code>.
     def inspect
-      sprintf("#<%s:%#0x URI:%s>", URI.to_s, self.object_id, self.to_s)
+      sprintf("#<%s:%#0x URI:%s>", self.class.to_s, self.object_id, self.to_s)
     end
 
     ##

spec/addressable/uri_spec.rb

@@ -1757,6 +1757,19 @@ def to_s
     expect(@uri.inspect).to include("%#0x" % @uri.object_id)
   end
 
+  context "when Addressable::URI has been sub-classed" do
+    class CustomURIClass < Addressable::URI
+    end
+
+    before do
+      @uri = CustomURIClass.parse("http://example.com")
+    end
+
+    it "when inspected, should have the sub-classes name" do
+      expect(@uri.inspect).to include("CustomURIClass")
+    end
+  end
+
   it "should use the 'http' scheme" do
     expect(@uri.scheme).to eq("http")
   end