use MethodParameter#isOptional() instead of checking for Optional type

[zarebski-m]

Check for whether a parameter is required now uses isOptional() method. This improves integration with JSR-305 annotations and Kotlin nullability.

In the following Kotlin example, parameter foo will be mandatory, but bar and baz not:

@RestController
@RequestMapping
class FooController {
    @GetMapping("test")
    fun testParams(
        @RequestParam foo: String,
        @RequestParam bar: String = "default",
        @RequestParam baz: String?
    ) {}
}

Similarly, parameter foo will correctly be rendered non-mandatory in this Java example:

@RestController
@RequestMapping
public class FooController {
    @GetMapping("test")
    void fooNullable(@RequestParam @Nullable String foo) {}
}

[bnasslahsen]

Thank you @zarebski-m for your contribution.