Avoid breaking keywords in labels.

[marcelbuesing]

Keywords such as ref previously lead to labels such as __#ref__value. This should be __ref__value.

[coveralls]

Pull Request Test Coverage Report for Build 226

• 10 of 11 (90.91%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 56.196%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
yaserde_derive/src/common/field.rs	0	1	0.0%
<!--	Total:	10	11	90.91%	-->

Totals
Change from base Build 225:	0.3%
Covered Lines:	907
Relevant Lines:	1614

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💛 - Coveralls