Fix a write buffer bug and more test cases.

Author: ginkgoch

src/writer.js

@@ -103,7 +103,8 @@ module.exports = class BufferWriter {
 
     writeBuffer(buffer) {
         assert(buffer, 'Buffer cannot be null.')
-        this.buffer = Buffer.concat([this.buffer, buffer])
+
+        buffer.copy(this.buffer, this.offset)
         this.offset += buffer.length
         return this.offset
     }

src/writer.test.js

@@ -23,16 +23,17 @@ describe('BufferWriter tests', () => {
         let offset = bw.writeBuffer(target1)
         expect(offset).toBe(6)
         expect(offset).toBe(bw.offset)
+        expect(buffer.length).toBe(256)
 
-        offset = bw.writeBuffer(target1)
+        offset = bw.writeBuffer(target2)
         expect(offset).toBe(12)
         expect(offset).toBe(bw.offset)
 
         const bufferSlice = buffer.slice(0, 12)
         const bufferConcat = Buffer.concat([target1, target2])
 
         expect(bufferConcat.length).toBe(12)
-        expect(Buffer.compare(bufferSlice, bufferConcat)).toBeTruthy()
+        expect(bufferConcat.equals(bufferSlice)).toBeTruthy()
     })
 
     it('demo 1', () => {