WIP: Parse dont validate

[dtkav]

Changes proposed in this pull request:

• move type coercion into one place (with validation)
• move sanitization to just before populating function arguments to avoid over-matching (like "$abc" and "abc$")
• refactor + simplify notorious _get_body_argument() function.

[coveralls]

Coverage increased (+0.1%) to 96.28% when pulling 4e63c82 on dtkav:parse_dont_validate into 4b93890 on zalando:master.

[cognifloyd]

I know this is WIP. I'm excited to see you separating parsing and validation. Your refactored code is much easier to follow.
One question about additionalProperites (it might just be not implemented yet because this PR is WIP). So, if this helps great. if not, just ignore. :)

[cognifloyd]

how does this handle an explicit additionalProperties: true?
It looks like it calls cast_leaves -> coerce_type so param ends up as a True bool, so param.get("schema", param) (line 54) should fail.
If additionalProperties is not defined or None (ie the default value of True applies), then this skips handling it which is different than an explict true.

Suggested change

	elif schema.get('additionalProperites'):
	elif schema.get('additionalProperites') and not isinstance(schema.get('additionalProperties'), bool):

But that doesn't handle cases where additionalProperties are just allowed, and no schema for them is defined.

[ThomasBendaSEL]

also spelling - 'additionalProperites' --> 'additionalProperties'

[RobbeSneyders]

This split is now done as part of #1489.