feat: utility to lowercase object properties

refs antongolub#33

- Adds utility that will help mimicing lowercasing of headers like it's done in express' Request object
- Code heavily inspired by https://github.com/howardabrams/node-mocks-http/blob/a0ad193459c102069a9b7de386f16ebf59cab6e0/lib/utils.js#L3-L9

src/main/js/util.js

@@ -18,3 +18,13 @@ export function appendAdditionalProps (target: Object, props: Object): void {
 export function concat (...strings: Array<?string>): string {
   return strings.join('')
 }
+
+export function convertKeysToLowerCase (map: Object): Object {
+  const newMap = {}
+
+  for (const key in map) {
+    newMap[key.toLowerCase()] = map[key]
+  }
+
+  return newMap
+}

src/test/js/util.js

@@ -0,0 +1,21 @@
+import { convertKeysToLowerCase } from '../../main/js/util'
+
+describe('util', () => {
+  describe('convertKeysToLowerCase', () => {
+    it('converts object keys to lower case keys', () => {
+      const map = {
+        prop: 'value',
+        standard_Notation: 42,
+        'BaZ-Header': 'qux'
+      }
+
+      const lowerCasedMap = convertKeysToLowerCase(map)
+
+      expect(map.standard_Notation).toEqual(42, 'original map is not modified')
+
+      expect(lowerCasedMap.prop).toEqual('value')
+      expect(lowerCasedMap.standard_notation).toEqual(42)
+      expect(lowerCasedMap['baz-header']).toEqual('qux')
+    })
+  })
+})