fix: use type_is_string to detect string type

[xiaoliu-heng]

fix issue #409

[chrisbeardy]

thanks for this, I've been meaning to look into this issue as i think it may be the same problem as #356 and #405. Since you're on the train of thought, could you check if it fixes these issues too (or the same solution is required), be good to close all 3 issues in one go!

[xiaoliu-heng]

yes, I have already tested it on the actual PLC hardware.

[xiaoliu-heng]

but it should only fixed the device notification

[coveralls]

Pull Request Test Coverage Report for Build 10521631021

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.926%

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Totals
Change from base Build 10177041600:	0.0%
Covered Lines:	1721
Relevant Lines:	1813

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💛 - Coveralls