fix(matrix): abs method should return the instance

src/matrix.js

@@ -1465,6 +1465,7 @@ Matrix.prototype.abs = function abs() {
             this[i][j] = Math.abs(this[i][j]);
         }
     }
+    return this;
 };
 
 module.exports = Matrix;

test/matrix/abs.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var Matrix = require('../..');
+
+describe('Matrix#abs', function () {
+
+    var matrix;
+
+    beforeEach(function () {
+        matrix = new Matrix([
+            [0, 1, 2],
+            [3, -4, -5],
+            [-6, -7, -8],
+            [4.39, -0.61, -12.7]
+        ]);
+    });
+
+    it('should convert all elements to absolute value (in-place)', function () {
+        matrix.abs();
+        matrix.should.eql([
+            [0, 1, 2],
+            [3, 4, 5],
+            [6, 7, 8],
+            [4.39, 0.61, 12.7]
+        ]);
+    });
+
+    it('should return instance', function () {
+        matrix.abs().should.equal(matrix);
+    });
+
+});