Implement unknownProperty in FastbootHeaders to prevent users from trolling

If a user mistakenly uses `Ember.get(headers, headerName)` it will work now, but will
warn the user to use `header.get(headerName)` instead.

Author: cibernox

package-lock.json

@@ -38,6 +38,7 @@
   "devDependencies": {
     "chai": "^4.1.0",
     "chai-as-promised": "^7.1.1",
+    "ember-source": "2.16.2",
     "express": "^4.15.4",
     "fs-promise": "^2.0.3",
     "mocha": "^4.0.0",

package.json

@@ -92,7 +92,7 @@ class FastBootHeaders {
   }
 
   unknownProperty(maybeHeader) {
-    console.warn(`You called \`Ember.get(headers, '${maybeHeader}')\` with a FastBootHeaders instance as first argument. FastBootHeader is not an Ember object and you should use \`headers.get('${maybeHeader}')\` instead.`)
+    console.warn(`You called \`Ember.get(headers, '${maybeHeader}')\` with a FastBootHeaders instance as first argument. FastBootHeader is not an Ember object and you should use \`headers.get('${maybeHeader}')\` instead.`);
     return this.get(maybeHeader);
   }
 }

src/fastboot-headers.js

@@ -148,7 +148,7 @@ describe('FastBootHeaders', function() {
     expect(entriesIterator.next()).to.deep.equal({ value: undefined, done: true });
   });
 
-  it('then used with mistakenly `Ember.get` with an unknown property, it attempts to get the header with that name and warns the user to use `headers.get` instead', function () {
+  it('when mistakenly used `Ember.get` with an unknown property, it attempts to get the header with that name and warns the user to use `headers.get` instead', function () {
     var headers = {
       'x-test-header': ['value1', 'value2']
     };