[#2299] bugfix(jakarta-ee): don't set character encoding to UTF-8 if it's alr…

[lprimak]

…eady set to UTF-8

Checks character encoding before setting it. Setting it when it's already set can generate a warning.
The prior check if it's already UTF-8 avoids the warning

fixes #2299
Documentation PR apache/shiro-site#242

Following this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GitHub issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a GitHub issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [#XXX] - Fixes bug in SessionManager,
  where you replace #XXX with the appropriate GitHub issue. Best practice
  is to use the GitHub issue title in the pull request title and in the first line of the commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• add fixes #XXX if merging the PR should close a related issue.
• Run mvn verify to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If you have a group of commits related to the same change, please squash your commits into one and force push your branch using git rebase -i.
• Committers: Make sure a milestone is set on the PR

Trivial changes like typos do not require a GitHub issue (javadoc, comments...).
In this case, just format the pull request title like [DOC] - Add javadoc in SessionManager.

If this is your first contribution, you have to read the Contribution Guidelines

If your pull request is about ~20 lines of code you don't need to sign an Individual Contributor License Agreement
if you are unsure please ask on the developers list.

To make clear that you license your contribution under the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004
• In any other case, please file an Apache Individual Contributor License Agreement.