fix(graphql): fixing the dart analyzer error

With the new version of the compiler, we get some
compiler error that makes the project unable to
complete with success all the checks are done via make.

This commit fixes these bugs and make the compiler happy again.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

packages/graphql/test/graphql_client_test.dart

@@ -150,7 +150,7 @@ void main() {
           equals(200),
         );
         expect(
-          r.context.entry<HttpLinkResponseContext>()!.headers['foo'],
+          r.context.entry<HttpLinkResponseContext>()!.headers!['foo'],
           equals('bar'),
         );
       });
@@ -548,7 +548,7 @@ query WalletGetContent($input: WalletGetContentInput!) {
           equals(200),
         );
         expect(
-          r.context.entry<HttpLinkResponseContext>()!.headers['foo'],
+          r.context.entry<HttpLinkResponseContext>()!.headers!['foo'],
           equals('bar'),
         );
       });
@@ -1443,7 +1443,7 @@ query WalletGetContent($input: WalletGetContentInput!) {
         equals(200),
       );
       expect(
-        r.context.entry<HttpLinkResponseContext>()!.headers['foo'],
+        r.context.entry<HttpLinkResponseContext>()!.headers!['foo'],
         equals('bar'),
       );
     });