Fix visitor label

[coveralls]

Pull Request Test Coverage Report for Build 197

• 11 of 18 (61.11%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 53.918%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
yaserde_derive/src/de/expand_struct.rs	0	3	0.0%
yaserde_derive/src/common/field.rs	0	4	0.0%
<!--	Total:	11	18	61.11%	-->

Files with Coverage Reduction	New Missed Lines	%
yaserde_derive/src/common/field.rs	1	0%
yaserde_derive/src/de/expand_struct.rs	1	0%
<!--	Total:	2		-->

Totals
Change from base Build 192:	0.2%
Covered Lines:	812
Relevant Lines:	1506

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💛 - Coveralls