commit, tag: return a single nil on unsigned objects

Instead of returning a tuple with nils, or the unsigned data, return a
single nil when the commit or tag is unsigned.

Author: carlosmn

ext/rugged/rugged_commit.c

@@ -625,7 +625,7 @@ static VALUE rb_git_commit_header(VALUE self)
 static VALUE rb_git_commit_extract_signature(int argc, VALUE *argv, VALUE self)
 {
 	int error;
-	VALUE ret_arr;
+	VALUE ret;
 	git_oid commit_id;
 	const char *field;
 	git_repository *repo;
@@ -648,18 +648,18 @@ static VALUE rb_git_commit_extract_signature(int argc, VALUE *argv, VALUE self)
 	}
 
 	if (error == GIT_ENOTFOUND && giterr_last()->klass == GITERR_OBJECT ) {
-		ret_arr = rb_ary_new3(2, Qnil, Qnil);
+		ret = Qnil;
 	} else {
 		rugged_exception_check(error);
 
-		ret_arr = rb_ary_new3(2, rb_str_new(signature.ptr, signature.size),
-				      rb_str_new(signed_data.ptr, signed_data.size));
+		ret = rb_ary_new3(2, rb_str_new(signature.ptr, signature.size),
+				     rb_str_new(signed_data.ptr, signed_data.size));
 	}
 
 	git_buf_free(&signature);
 	git_buf_free(&signed_data);
 
-	return ret_arr;
+	return ret;
 }
 
 void Init_rugged_commit(void)

lib/rugged/tag.rb

@@ -15,7 +15,7 @@ def self.extract_signature(repo, oid, prefix=GPG_SIGNATURE_PREFIX)
           object.data.byteslice(0...index)
         ]
       else
-        [nil, object.data]
+        nil
       end
     end
 

test/commit_test.rb

@@ -272,7 +272,7 @@ def test_extract_signature
     assert_equal exp_signature, signature
     assert_equal exp_signed_data, signed_data
 
-    assert_equal [nil, nil], Rugged::Commit.extract_signature(@repo, "8496071c1b46c854b31185ea97743be6a8774479")
+    assert_nil Rugged::Commit.extract_signature(@repo, "8496071c1b46c854b31185ea97743be6a8774479")
 
     # Ask for a tree
     assert_raises(Rugged::InvalidError) { Rugged::Commit.extract_signature(@repo, "181037049a54a1eb5fab404658a3a250b44335d7") }