fix draw function can only call once

[vaniship]

After working around the first problem was, that the diagram was only rendered the first time. Each time after that cannot read property x of undefined occurred.

I fix this bug. idCache not clear when then next draw.

fix #748

[coveralls]

Pull Request Test Coverage Report for Build 751

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 54.246%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/diagrams/class/classRenderer.js	1	2	50.0%

Totals
Change from base Build 716:	-0.01%
Covered Lines:	2051
Relevant Lines:	3756

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💛 - Coveralls