Make duck-typing checking in serialize_body method more restrictive

[piglei]

Thebody parameter in DynamicClient's most methods currently accepts both dict type and ResourceInstance type. This was achieved by calling serialize_body() before mading any requests.

But in current implementation, the duck-typing style checking in serialize_body was kind of meaning-less---ResourceInstance type has overwritten it's __getattr__ method, so basically it's instances can pass every hasattr(...) check.

>>> inst = ResourceInstance(None, {'kind': 'Pod'})
>>> hasattr(inst, 'to_dict')
True
>>> hasattr(inst, 'to_dict_whatever_name')
True

This PR makes the checking in serialize_body() method more restrictive and also add some related unit tests.

Release Note:

Type checking in `Client.serialize_body()` was made more restrictive and robust.

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[roycaihw]

/assign @yliaog

[yliaog]

/lgtm

/assign @fabianvf

[fabianvf]

/lgtm, needs rebase though

[yliaog]

/approve

[piglei]

/lgtm, needs rebase though

Rebased with upstream/master.

[piglei]

PR refreshed.

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[yliaog]

please squash the commits

[piglei]

please squash the commits

Done

[yliaog]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabianvf, piglei, yliaog

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[fabianvf]

@piglei just needs the release note and it should be good to go

[piglei]

@piglei just needs the release note and it should be good to go

Thanks for reminding me, the release note was now added.