createKey() override (#41)

Look for a createKey function on the Model prototype and use it if found.  Otherwise, use the createKey function found in koop-core.

CHANGELOG.md

@@ -3,6 +3,9 @@ All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
+### Added
+* Prefentially use a `createKey` function found on the Model's prototype
+
 ## Fixed
 * Apply standard.js to source and test files
 * Make style adjustments to pass standard.js

src/models/index.js

@@ -3,7 +3,7 @@ function Model (options) {
 }
 
 Model.prototype.pull = function (req, callback) {
-  const key = createKey(req)
+  const key = (this.createKey) ? this.createKey(req) : createKey(req)
   this.cache.retrieve(key, req.query, (err, cached) => {
     if (!err && isFresh(cached)) {
       callback(null, cached)