Remove 'binary' encoding assert - add tests

Don't write large characters to buffers with binary encoding. You will be
silently injured.

src/node.cc

@@ -1203,7 +1203,6 @@ ssize_t DecodeWrite(char *buf,
 
   for (size_t i = 0; i < buflen; i++) {
     unsigned char *b = reinterpret_cast<unsigned char*>(&twobytebuf[i]);
-    assert(b[1] == 0);
     buf[i] = b[0];
   }
 

test/simple/test-buffer.js

@@ -505,3 +505,18 @@ assert.equal(0x6f, z[1]);
 var b = new SlowBuffer(10);
 b.write('あいうえお', 'ucs2');
 assert.equal(b.toString('ucs2'), 'あいうえお');
+
+// Binary encoding should write only one byte per character.
+var b = Buffer([0xde, 0xad, 0xbe, 0xef]);
+var s = String.fromCharCode(0xffff);
+b.write(s, 0, 'binary')
+assert.equal(0xff, b[0]);
+assert.equal(0xad, b[1]);
+assert.equal(0xbe, b[2]);
+assert.equal(0xef, b[3]);
+s = String.fromCharCode(0xaaee);
+b.write(s, 0, 'binary')
+assert.equal(0xee, b[0]);
+assert.equal(0xad, b[1]);
+assert.equal(0xbe, b[2]);
+assert.equal(0xef, b[3]);