PDFReader Test `doesNotContains` -> `contain` and Replace `Collectors.joining()` to `String.join()`

[rivkode]

Expected Behavior

• Using of assertThat().doesNotContain() -> assertThat().contains() may be passed for the test.
• Using of String.join() may be for readability and simplicity.

When I looked at the other codes, most of them were using Collectors.joining(), with one exception. (RedisFilterExpressionConverter)
So I will change the modification to String.join() back to Collectors.joining().
If you think I should change it to String.join(), please comment and I will change it.

Current Behavior

I was wondering in the pdf file there was text page 1 of 4, page of 2 of 4 ... but why using doesNotContain() for the same text?
(If I misunderstand I will change it.)

I changed it to String.join because I don't think we need the extra stream operation for the simple joining of strings of text in a PDF file.
(If it is necessary, I will change it.)

Context

• I modifieddoesNotContain() to contain. Then the test will be a success.
• I modified Collectors.joining() to String.join().

Please see PR as below.

• Replace doesNotContains and Collectors.joining() #754

[markpollack]

Appreciate the efforts @rivkode Closing the issue now.