login: Send trimmed username/email in fetch-API-key request, using .trim()

Fixes: zulip#547

[chris: edited commit message to match style; adjusted blank lines
and indentation in test for consistency with neighboring tests]

Author: Mehul-Kumar-27

lib/widgets/login.dart

@@ -490,7 +490,7 @@ class _UsernamePasswordFormState extends State<_UsernamePasswordForm> {
     if (!usernameValid || !passwordValid) {
       return;
     }
-    final String username = usernameFieldState.value!;
+    final String username = usernameFieldState.value!.trim();
     final String password = passwordFieldState.value!;
 
     widget.loginPageState.setState(() {

test/widgets/login_test.dart

@@ -135,6 +135,26 @@ void main() {
             id: testBinding.globalStore.accounts.single.id));
       });
 
+      testWidgets('trims whitespace on username', (tester) async {
+        final serverSettings = eg.serverSettings();
+        await prepare(tester, serverSettings);
+        check(testBinding.globalStore.accounts).isEmpty();
+
+        await tester.enterText(findUsernameInput, '  ${eg.selfAccount.email}  ');
+        await tester.enterText(findPasswordInput, 'p455w0rd');
+        connection.prepare(json: FetchApiKeyResult(
+          apiKey: eg.selfAccount.apiKey,
+          email: eg.selfAccount.email,
+          userId: eg.selfAccount.userId,
+        ).toJson());
+        await tester.tap(findSubmitButton);
+        checkFetchApiKey(username: eg.selfAccount.email, password: 'p455w0rd');
+        await tester.idle();
+        check(testBinding.globalStore.accounts).single
+          .equals(eg.selfAccount.copyWith(
+            id: testBinding.globalStore.accounts.single.id));
+      });
+
       testWidgets('account already exists', (tester) async {
         final serverSettings = eg.serverSettings();
         await prepare(tester, serverSettings);