Cloning and Expression of an Cytosolic Ascorbate Peroxidase Gene from Populus tomentosa Characteristics of Peroxidase from Agaricus bisporus and Its Relationship with Browning Characteristics of Peroxidase from Agaricus bisporus and Its Relationship with Browning Preparation of Cellulose-Based Immobilized Enzyme Matrix and the Exploration of Immobilized Laccase Performance