Request body should allow Buffer object

[aleclarson]

…and it shouldn't assume UTF8 encoding.

Currently, when calculating the content hash, UTF8 encoding is assumed:

aws4/aws4.js

Lines 14 to 15 in a413aad

	function hash(string, encoding) {
	return crypto.createHash('sha256').update(string, 'utf8').digest(encoding)

[sspiff]

According to https://nodejs.org/api/crypto.html#hashupdatedata-inputencoding, hash.update() will ignore the given encoding ('utf8' here) when the given data (string here) is already a Buffer. So maybe it already admits body as a Buffer? Admittedly, I haven't tested this.