implement _serializeKey and _serializeValue

With undefined behavior for nullish targets (range options and
seek targets). Previously, nullish range options were ignored and
seek(null) and seek(undefined) would throw an error. Now they
translate to String(null) and String(undefined).

This change makes it explicit that leveldown only supports buffers
and strings. Nullish targets do have a meaning in the ecosystem;
that meaning should be given at a higher level like encoding-down.

In other words, it isn't leveldown's concern anymore.

leveldown.js

@@ -29,6 +29,14 @@ LevelDOWN.prototype._close = function (callback) {
   this.binding.close(callback)
 }
 
+LevelDOWN.prototype._serializeKey = function (key) {
+  return Buffer.isBuffer(key) ? key : String(key)
+}
+
+LevelDOWN.prototype._serializeValue = function (value) {
+  return Buffer.isBuffer(value) ? value : String(value)
+}
+
 LevelDOWN.prototype._put = function (key, value, options, callback) {
   this.binding.put(key, value, options, callback)
 }

test/iterator-test.js

@@ -4,7 +4,7 @@ const lexi = require('lexicographic-integer')
 const util = require('util')
 
 make('iterator#seek throws if target is empty', function (db, t, done) {
-  var targets = [null, undefined, '', Buffer.alloc(0), []]
+  var targets = ['', Buffer.alloc(0), []]
   var pending = targets.length
 
   targets.forEach(function (target) {