Fix model serialization

[rakshith91]

msrest serializes objects as a string rather than dictionary

Please look at Azure/azure-sdk-for-python#12831

class CustomSample(msrest.serialization.Model):
    _validation = {
            'a': {'required': True},
    }

    _attribute_map = {
        'a': {'key': 'a', 'type': 'str'},
    }

    def __init__(self, a):
        self.a = a

would be serialized as 'data': "{'a': 'sample event'}", (as an instance of string rather than dict)

Fixes Azure/azure-sdk-for-python#12831

[codecov-commenter]

Codecov Report

Merging #220 into master will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   86.22%   86.06%   -0.16%     
==========================================
  Files          25       25              
  Lines        2736     2677      -59     
==========================================
- Hits         2359     2304      -55     
+ Misses        377      373       -4     

Impacted Files	Coverage Δ
msrest/serialization.py	90.80% <100.00%> (-0.30%)	⬇️
msrest/configuration.py	75.86% <0.00%> (-2.27%)	⬇️
msrest/universal_http/requests.py	82.01% <0.00%> (-0.74%)	⬇️
msrest/paging.py	91.22% <0.00%> (-0.30%)	⬇️
msrest/exceptions.py	87.34% <0.00%> (-0.16%)	⬇️
msrest/universal_http/__init__.py	74.09% <0.00%> (-0.15%)	⬇️
msrest/pipeline/universal.py	94.11% <0.00%> (-0.12%)	⬇️
msrest/pipeline/__init__.py	91.20% <0.00%> (+0.57%)	⬆️
msrest/pipeline/async_abc.py	91.42% <0.00%> (+1.28%)	⬆️
... and 1 more

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