Add readBodyBuffer method to HttpClientResponse (#1475)

* Add readBodyBuffer method to HttpClientResponse

* Implement method in other package tests

* Make method optional to satisfy the test process

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@actions/http-client",
-  "version": "2.1.0",
+  "version": "2.1.1",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "@actions/http-client",
-      "version": "2.1.0",
+      "version": "2.1.1",
       "license": "MIT",
       "dependencies": {
         "tunnel": "^0.0.6"