Avoid unnecessary String concatenations in StompHeaderAccessor

[dreis2211]

Closes SPR-14625

[pivotal-issuemaster]

@dreis2211 Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[sbrannen]

As I mentioned in the corresponding JIRA issue...

FYI: on Spring 5.0, the proposed changes can be rewritten as lambda expressions as follows.

Assert.state(SimpMessageType.MESSAGE.equals(getMessageType()), () -> "Unexpected message type " + getMessage());

Assert.state(SimpMessageType.MESSAGE.equals(getMessageType()), () -> "Unexpected message type " + getMessage());

If backported to Spring 4.x, however, the use of an if-block will still be required.

[jhoeller]

Thanks again for the pull request! Applied in a revised form.