Buffers and strings don't encode the same as quoted-printable

[gene-hightower]

I suspect that Buffers and strings should encode the same way. A few new tests will fail:

diff --git a/test/utils.js b/test/utils.js
index 73dfb55..5be73f9 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -38,6 +38,13 @@ describe('encode_qp', function () {
         done();
     })

+    it('plain ascii should not be encoded in Buffers', function (done) {
+        assert.equal(
+            utils.encode_qp(Buffer.from('quoted printable')),
+            'quoted printable');
+        done();
+    })
+
     it('8-bit chars should be encoded', function (done) {
         assert.equal(
             utils.encode_qp(
@@ -47,20 +54,29 @@ describe('encode_qp', function () {
         done();
     })

+    it('8-bit chars should be encoded in Buffers', function (done) {
+        assert.equal(
+            utils.encode_qp(
+                Buffer.from('v\xe5re kj\xe6re norske tegn b\xf8r \xe6res')
+            ),
+            'v=C3=A5re kj=C3=A6re norske tegn b=C3=B8r =C3=A6res');
+        done();
+    })
+