feat(Fetch) support keep-alive connections

[fupduck]

Currently keep-alive connections throw an IllegalStateException - already connected

This adds keep-alive support, by catching errors if already connected and not setting Content-Length explicitly.

[coveralls]

Pull Request Test Coverage Report for Build 106

• 13 of 14 (92.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 98.901%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nodes/src/main/java/io/aexp/nodes/graphql/Fetch.java	13	14	92.86%
<!--	Total:	13	14	92.86%	-->

Totals
Change from base Build 104:	-0.2%
Covered Lines:	540
Relevant Lines:	546

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💛 - Coveralls