#1084: decode body if base64 is enable

[npeters]

Issue #1084:

Description of changes:
if isBase64Encoded == true then AwsStringHttpProcessingUtils must decode the body

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[npeters]

by default the content-type must be application/json

[deki]

Thank you very much for your contribution. Agree that this needs to be fixed. Could you reduce the duplicated code?
We plan to have a release early next week and it would be great to include it.

[deki]

Thank you very much for your contribution, @npeters! The changes have been merged and will be part of the next release.