Refactor ContentDisposition type check

[Tussle0410]

I noticed that there was duplicated logic when checking the definitions of DispositionType in ContentDisposition.java, so I extracted this logic into a private method.

After that, I realized there weren't any tests to verify this code was working properly, so I went ahead and added test code for it.

    public boolean isAttachment() {
	return (this.type != null && this.type.equalsIgnoreCase("attachment"));
    }
    ...

I've modified the existing code to follow the format shown below.

    public boolean isAttachment() {
	return isDispositionType("attachment");
    }

    private boolean isDispositionType(String type) {
	return this.type != null && this.type.equalsIgnoreCase(type);
    }

[bclozel]

Is this a typo? Did you mean isDispositionType?

[Tussle0410]

I made a mistake. There's a typo. I meant isDispositionType. I'll fix it.

[bclozel]

Thanks for your contribution @Tussle0410 , this is now merged in the main branch.