Support base64 encoding of bytes input for JSON blob types.

[danielgtaylor]

This pull request modifies the code added in ffcef5f to also support
arguments passed in as bytes, such as those you might encounter when
using KMS.

See this related issue: boto/boto3#31

import boto3
kms = boto3.client('kms')
result = kms.encrypt(KeyId='some_key_id_redacted',Plaintext='my name is bob')
kms.decrypt(CiphertextBlob=result['CiphertextBlob'])

Since the JSON data for serialization tests doesn't support specifying
bytes as input, I've added a new test class to cover this case.

cc @jamesls, @kyleknap

[jamesls]

We are now no longer consistently handling blob types across protocols, as this logic is only in the JSONSerializer, and this is now duplicated from the query protocol. I think we should refactor this so that all the protocols can share a similar blob serializer. There's even a TODO note in the RestXML serializer specifically about this point.

[jamesls]

+1 to handling this case, but we should port this over to all protocols so they all have the same behavior regarding bytes types.

[danielgtaylor]

I've updated / refactored the code such that all serializers now use the same shared logic to convert a value to base64. Please take another look @jamesls!

[jamesls]

Looks good.

[kyleknap]

LGTM 🚢