Fix Book form (create) returning to the full books list on cancel

Fixes #1662
Added a small block of logic to determine the correct URL to attribute to the cancel button on a given page create form.
If adding a book from a bookshelf, return to the bookshelf. If editing a book, return to the book. In all other cases, return to the full books list.

tests/Entity/EntityTest.php

@@ -1,5 +1,6 @@
 <?php namespace Tests;
 
+use BookStack\Entities\Bookshelf;
 use BookStack\Entities\Book;
 use BookStack\Entities\Chapter;
 use BookStack\Entities\Page;
@@ -292,4 +293,29 @@ class EntityTest extends BrowserKitTest
         $this->assertEquals('parta-partb-partc', $book->slug);
     }
 
+    public function test_shelf_cancel_creation_returns_to_correct_place()
+    {
+        $shelf = Bookshelf::first();
+
+        // Cancel button from shelf goes back to shelf
+        $this->asEditor()->visit($shelf->getUrl('/create-book'))
+            ->see('Cancel')
+            ->click('Cancel')
+            ->seePageIs($shelf->getUrl());
+
+        // Cancel button from books goes back to books
+        $this->asEditor()->visit('/create-book')
+            ->see('Cancel')
+            ->click('Cancel')
+            ->seePageIs('/books');
+
+        // Cancel button from book edit goes back to book
+        $book = Book::first();
+
+        $this->asEditor()->visit($book->getUrl('/edit'))
+            ->see('Cancel')
+            ->click('Cancel')
+            ->seePageIs($book->getUrl());
+    }
+
 }