Add "type" to EXCLUDE_KEYS in blocks serializer/deserializer

[cekk]

This is needed to avoid code to convert "type" slate attribute value into an internal link.

slate set something like type: "link" and i had an edge-case where "link" was a content-id, so the attribute was converted in something like:

type:"http://mysite.com/link"

[coveralls]

Pull Request Test Coverage Report for Build 10705043478

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 66.535%

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Totals
Change from base Build 10680708577:	0.0%
Covered Lines:	1008
Relevant Lines:	1515

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💛 - Coveralls