Preserve request headers across multiple requests

It is customary in many real-world test suites to do something like

   browser.addHeader('Authorization', 'mgr:mgrpw')

and then expect that to take effect for all subsequent browser.open(...),
browser.getLink(...).click(), browser.getControl(...).submit() calls.

src/zope/testbrowser/browser.py

@@ -446,7 +446,6 @@ def _findAllControls(self, forms, include_subcontrols=False):
     def _changed(self):
         self._counter += 1
         self._contents = None
-        self._req_headers = {}
         self._controls = {}
         self.__html = None
 
@@ -469,6 +468,7 @@ def _preparedRequest(self, url):
             extra_environ['wsgi.handleErrors'] = False
             extra_environ['paste.throw_errors'] = True
             extra_environ['x-wsgiorg.throw_errors'] = True
+            self._req_headers.pop('X-zope-handle-errors', None)
 
         kwargs = {'headers': sorted(self._req_headers.items()),
                   'extra_environ': extra_environ,

src/zope/testbrowser/tests/test_wsgi.py

@@ -101,13 +101,17 @@ def test_handle_errors(self):
         self.assertEqual(browser.contents, 'None')
         browser.open('http://localhost/echo_one.html?var=wsgi.handleErrors')
         self.assertEqual(browser.contents, 'None')
+        browser.open('http://localhost/echo_one.html?var=HTTP_X_ZOPE_HANDLE_ERRORS')
+        self.assertEqual(browser.contents, "'True'")
         browser.handleErrors = False
         browser.open('http://localhost/echo_one.html?var=x-wsgiorg.throw_errors')
         self.assertEqual(browser.contents, 'True')
         browser.open('http://localhost/echo_one.html?var=paste.throw_errors')
         self.assertEqual(browser.contents, 'True')
         browser.open('http://localhost/echo_one.html?var=wsgi.handleErrors')
         self.assertEqual(browser.contents, 'False')
+        browser.open('http://localhost/echo_one.html?var=HTTP_X_ZOPE_HANDLE_ERRORS')
+        self.assertEqual(browser.contents, 'None')
 
     def test_non_ascii_urls(self):
         teststr = u'~ひらがな'
@@ -173,12 +177,15 @@ def test_unwanted_headers(self):
         self.unwrapped_browser.open(url)
         self.assertTrue('x-powered-by' in self.unwrapped_browser.headers)
         self.assertTrue('x-content-type-warning' in self.unwrapped_browser.headers)
-    
+
     def test_authorization(self):
         # Basic authorization headers are encoded in base64
         self.browser.addHeader('Authorization', 'Basic mgr:mgrpw')
         self.browser.open('http://localhost/echo_one.html?var=HTTP_AUTHORIZATION')
         self.assertEqual(self.browser.contents, repr('Basic bWdyOm1ncnB3'))
+        # this header persists over multiple requests
+        self.browser.open('http://localhost/echo_one.html?var=HTTP_AUTHORIZATION')
+        self.assertEqual(self.browser.contents, repr('Basic bWdyOm1ncnB3'))
 
     def test_authorization_other(self):
         # Non-Basic authorization headers are unmolested