Fixed handle_decimals to raise a TypeError if it can't encode the object

[subssn21]

Issue #, if available:

Description of changes:
default Json Encoders should raise a TypeError if they can't properly encode an object.

At the moment when you try to encode an object that is not JSON serializable you get:
[ERROR] ValueError: Circular reference detected
Traceback (most recent call last):
File "/var/task/chalice/app.py", line 820, in call
response = response.to_dict(self.api.binary_types)
File "/var/task/chalice/app.py", line 352, in to_dict
default=handle_decimals)
File "/var/lang/lib/python3.7/json/init.py", line 238, in dumps
**kw).encode(obj)
File "/var/lang/lib/python3.7/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/var/lang/lib/python3.7/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)

Now you get:
[ERROR] TypeError: Object of type dict_values is not JSON serializable
Traceback (most recent call last):
File "/var/task/chalice/app.py", line 820, in call
response = response.to_dict(self.api.binary_types)
File "/var/task/chalice/app.py", line 352, in to_dict
default=handle_decimals)
File "/var/lang/lib/python3.7/json/init.py", line 238, in dumps
**kw).encode(obj)
File "/var/lang/lib/python3.7/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/var/lang/lib/python3.7/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)
File "/var/task/chalice/app.py", line 55, in handle_decimals
raise TypeError('Object of type %s is not JSON serializable' % obj.class.name)

which conforms to what Python expects:

https://docs.python.org/2/library/json.html#json.JSONEncoder

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #1100 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1100      +/-   ##
==========================================
+ Coverage   95.47%   95.52%   +0.04%     
==========================================
  Files          27       27              
  Lines        4577     4577              
  Branches      579      579              
==========================================
+ Hits         4370     4372       +2     
+ Misses        135      134       -1     
+ Partials       72       71       -1

Impacted Files	Coverage Δ
chalice/app.py	97.31% <100%> (+0.31%)	⬆️

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[stealthycoin]

Thank you for the contribution, I pushed a changelog commit, and changed the name of a function to make more sense in the context of the stack trace it generates.

New PR: #1117.