Fix merging of Buffer objects

Previously, if you attempted to `build`/`create` an object whose properties contained `Buffer`s, they would be converted from the native `Buffer` type into an array (and thus creating an _incorrect_ value). This is caused by the use of lodash's `merge` implementation (see lodash/lodash#2964).

The recommended solution from the lodash project is to define a merge `customizer` to specifically handle this situation, which is what we do here.

lib/merge.ts

@@ -7,7 +7,7 @@ export const mergeCustomizer = (
   key: 'string',
   object: any,
 ) => {
-  if (Array.isArray(srcVal)) {
+  if (Array.isArray(srcVal) || srcVal instanceof Buffer) {
     return srcVal;
   } else if (srcVal === undefined) {
     object[key] = srcVal;