Copy headers (shallow) so the original is not modified

Fixes #164

Also update mocha to 10.4

aws4.js

@@ -46,7 +46,7 @@ function RequestSigner(request, credentials) {
 
   if (typeof request === 'string') request = url.parse(request)
 
-  var headers = request.headers = (request.headers || {}),
+  var headers = request.headers = Object.assign({}, (request.headers || {})),
       hostParts = (!this.service || !this.region) && this.matchHost(request.hostname || request.host || headers.Host || headers.host)
 
   this.request = request

package.json

@@ -15,7 +15,7 @@
     "integration": "node ./test/slow.js"
   },
   "devDependencies": {
-    "mocha": "^10.2.0",
+    "mocha": "^10.4.0",
     "should": "^13.2.3"
   }
 }

test/fast.js

@@ -274,6 +274,27 @@ describe('aws4', function() {
     })
   })
 
+  describe('#sign() with copies', function() {
+    it('should modify request/opts in place and return same object', function() {
+      var opts = {service: 'sqs'}
+      var newOpts = aws4.sign(opts)
+      opts.should.equal(newOpts)
+    })
+
+    it('should not modify existing headers', function() {
+      var headers = {
+        'Content-Type': 'application/x-amz-json-1.0',
+        'X-Amz-Target': 'DynamoDB_20120810.ListTables',
+      }
+      var opts = aws4.sign({
+        service: 'dynamodb',
+        headers: headers,
+        body: '{}',
+      })
+      opts.headers.should.not.equal(headers)
+    })
+  })
+
   describe('#sign() with signQuery', function() {
     it('should work with standard services', function() {
       var opts = aws4.sign({