Separate commonly used logic into methods and add test code

[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 isDepositionType("attachment");
	}

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