Fixing default transformRequest with buffer pools (#1511)

* Fixing default transformRequest of TypedArrays with buffer pools

A buffer pool is a large ArrayBuffer of a preset size used with a TypedArray
such as Uint8Array. This can speed up performance when constructing TypedArrays
of unknown sizes, and is a technique used by Node with their Buffers, and
by libraries like dcodeIO/protobuf.js.

Because the ArrayBuffer of such a TypedArray is much longer than the array
itself, using `.buffer` to transform the array before POSTing results in
sending a request with many extraneous empty bytes, which is wastefule and may
result in unexpected behavior.

Using `.slice()` before grabbing the ArrayBuffer fixes the problem by creating
a new TypedArray with a buffer of the expected length.

Signed-off-by: Zac Delventhal <delventhalz@gmail.com>

* Adding test for using default transformRequest with buffer pools

Adds a new test to the default transformRequest, running it on a
Uint8Array with a byte length of 16, but a much larger ArrayBuffer
with a byte length of 256. The transformed array should not include
any extra bytes, and so must have a byte length of just 16.

Signed-off-by: Zac Delventhal <delventhalz@gmail.com>

Co-authored-by: Zac Delventhal <zac@bitwise.io>
Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/defaults.js

@@ -41,7 +41,7 @@ var defaults = {
       return data;
     }
     if (utils.isArrayBufferView(data)) {
-      return data.buffer;
+      return data.slice().buffer;
     }
     if (utils.isURLSearchParams(data)) {
       setContentTypeIfUnset(headers, 'application/x-www-form-urlencoded;charset=utf-8');