AVRO-2401:Initializing decimal fields with fixed can casue padding issues

[zeshuai007]

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[RyanSkraba]

Hmmm -- this changes the behaviour for all fixed types with schemas, not just Decimal logical types. I'm not sure what that impact would be.

I'd be tempted to think that the right thing to do is to just throw an exception if the default size doesn't match the schema in terms of size... I know we tightened up the requirements for defaults in 1.9.x. What do you think?