Don't return 400 when read-only property is provided

[RobbeSneyders]

Fixes #942

No longer return 400 if a read-only property is provided, as discussed in the issue. We still raise an error if write-only properties are included in the response and response validation is enabled.

I also changed how read-/write-only works in combination with required. Previously, required would not be overwritten by read-/write-only. Now we just follow the spec to the letter:

• required and read-only: must be included but must be ignored by the server
• required and write-only: impossible to achieve, but I also don't see how this combination could make sense
• read-only: may be included but must be ignored by server
• write-only: must not be included by server

[coveralls]

Pull Request Test Coverage Report for Build 4308996494

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 92.347%

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Totals
Change from base Build 4270694070:	0.02%
Covered Lines:	3258
Relevant Lines:	3528

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