buffer: fix case of one buffer passed to concat

Fix Buffer.concat() so a copy is always returned regardless of the
number of buffers that were passed.

Previously if the array length was one then the same same buffer was
returned. This created a special case for the user where there was a
chance mutating the buffer returned by .concat() could mutate the buffer
passed in.

Also fixes an inconsistency when throwing if an array member was not a
Buffer instance. For example:

    Buffer.concat([42]);      // Returns 42
    Buffer.concat([42, 1]);  // Throws a TypeError

Now .concat() will always throw if an array member is not a Buffer
instance.

See: #1891
PR-URL: #1937
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/buffer.markdown

@@ -130,11 +130,6 @@ the list together.
 If the list has no items, or if the totalLength is 0, then it returns a
 zero-length buffer.
 
-If the list has exactly one item, then the first item of the list is
-returned.
-
-If the list has more than one item, then a new Buffer is created.
-
 If totalLength is not provided, it is read from the buffers in the list.
 However, this adds an additional loop to the function, so it is faster
 to provide the length explicitly.

lib/internal/buffer_new.js

@@ -183,8 +183,6 @@ Buffer.concat = function(list, length) {
 
   if (list.length === 0)
     return new Buffer(0);
-  else if (list.length === 1)
-    return list[0];
 
   if (length === undefined) {
     length = 0;

lib/internal/buffer_old.js

@@ -249,8 +249,6 @@ Buffer.concat = function(list, length) {
 
   if (list.length === 0)
     return new Buffer(0);
-  else if (list.length === 1)
-    return list[0];
 
   if (length === undefined) {
     length = 0;

test/parallel/test-buffer-concat.js

@@ -14,8 +14,14 @@ var flatLongLen = Buffer.concat(long, 40);
 
 assert(flatZero.length === 0);
 assert(flatOne.toString() === 'asdf');
-assert(flatOne === one[0]);
+// A special case where concat used to return the first item,
+// if the length is one. This check is to make sure that we don't do that.
+assert(flatOne !== one[0]);
 assert(flatLong.toString() === (new Array(10 + 1).join('asdf')));
 assert(flatLongLen.toString() === (new Array(10 + 1).join('asdf')));
 
+assert.throws(function() {
+  Buffer.concat([42]);
+}, TypeError);
+
 console.log('ok');