Don't include ellipsis in truncated digest output

Using truncate to limit the length of the digest has the unwanted side
effect of adding an ellipsis when the input is longer than the limit.

Also don't instantiate a new object for every digest.

Inspired by
rails/rails@b9e7c67

lib/deface/digest.rb

@@ -10,16 +10,8 @@ def digest_class=(klass)
       end
 
       def hexdigest(arg)
-        new.hexdigest(arg)
+        @digest_class.hexdigest(arg)[0...32]
       end
     end
-
-    def initialize(klass = nil)
-      @digest_class = klass || self.class.digest_class
-    end
-
-    def hexdigest(arg)
-      @digest_class.hexdigest(arg).truncate(32)
-    end
   end
 end

spec/deface/digest_spec.rb

@@ -4,20 +4,31 @@
 
 module Deface
   describe Digest do
+    def with_custom_digest
+      digest = double("digest")
+      original_digest = Digest.digest_class
+      Digest.digest_class = digest
+      yield digest
+    ensure
+      Digest.digest_class = original_digest
+    end
+
     it "should use MD5 by default" do
-      expect(Digest.new.hexdigest("123")).to eq "202cb962ac59075b964b07152d234b70"
+      expect(Digest.hexdigest("123")).to eq "202cb962ac59075b964b07152d234b70"
     end
 
     it "should use user-provided digest" do
-      digest = double("digest")
-      expect(digest).to receive(:hexdigest).with("to_digest").and_return("digested")
-      expect(Digest.new(digest).hexdigest("to_digest")).to eq "digested"
+      with_custom_digest do |digest|
+        expect(digest).to receive(:hexdigest).with("to_digest").and_return("digested")
+        expect(Digest.hexdigest("to_digest")).to eq "digested"
+      end
     end
 
     it "should truncate digest to 32 characters" do
-      digest = double("digest")
-      expect(digest).to receive(:hexdigest).with("to_digest").and_return("a" * 50)
-      expect(Digest.new(digest).hexdigest("to_digest").size).to eq 32
+      with_custom_digest do |digest|
+        expect(digest).to receive(:hexdigest).with("to_digest").and_return("a" * 50)
+        expect(Digest.hexdigest("to_digest").size).to eq 32
+      end
     end
   end
 end