Ensure default options do not mutate

Lodash's merge function mutates the first object,
this was setting defaults where we didn't want them.

This change makes sure the merging is happening to a new object, so that tests do not mutate each other.

__snapshots__/index.test.js.snap

@@ -55,5 +55,21 @@ Received:
 
 \\"Form elements must have labels (label)\\"
 
-Try fixing it with this help: https://dequeuniversity.com/rules/axe/2.6/label?application=axeAPI"
+Try fixing it with this help: https://dequeuniversity.com/rules/axe/2.6/label?application=axeAPI
+
+────────
+
+Expected the HTML found at $('body > a[href$=\\"#link-name\\"]') to have no violations:
+
+<a href=\\"#link-name\\"></a>
+
+Expected the HTML found at $('body > a[href$=\\"#link-name-2\\"]') to have no violations:
+
+<a href=\\"#link-name-2\\"></a>
+
+Received:
+
+\\"Links must have discernible text (link-name)\\"
+
+Try fixing it with this help: https://dequeuniversity.com/rules/axe/2.6/link-name?application=axeAPI"
 `;

index.js

@@ -32,7 +32,7 @@ function configureAxe (defaultOptions = {}) {
     // aXe requires real Nodes so we need to inject into Jests' jsdom document.
     document.body.innerHTML = html
 
-    const options = merge(defaultOptions, additionalOptions)
+    const options = merge({}, defaultOptions, additionalOptions)
 
     return new Promise((resolve, reject) => {
       axeCore.run(document.body, options, (err, results) => {

index.test.js

@@ -91,6 +91,35 @@ describe('jest-axe', () => {
         axe('Hello, World')
       }).toThrow('html parameter ("Hello, World") has no elements')
     })
+
+    it('should not mutate previous options', async () => {
+      let results = await axe(failingHtmlExample, {
+        rules: {
+          'link-name': { enabled: false }
+        }
+      })
+      let violations = results.violations
+      expect(violations.length).toBe(0)
+
+      const configuredAxe = configureAxe({
+        rules: {
+          'link-name': { enabled: false }
+        }
+      })
+
+      results = await configuredAxe(failingHtmlExample, {
+        rules: {
+          'link-name': { enabled: false }
+        }
+      })
+      violations = results.violations
+      expect(violations.length).toBe(0)
+
+      results = await axe(failingHtmlExample)
+      const violation = results.violations[0]
+      expect(violation.id).toBe('link-name')
+      expect(violation.description).toBe('Ensures links have discernible text')
+    })
   })
   describe('toHaveNoViolations', () => {
     const failingAxeResults = {