Support serializing "fixed" fields

Hi again, Tatu/multifarious/FasterXML!

I simply added a check for Schema.Type.Fixed in ObjectWriteContext, and massaged the object passed to GenericRecord as needed. Note that I suspect similar adjustments may be necessary for maps/arrays/etc, but I haven't had time to look into it. Perhaps there's a more natural way to do this?

Beyond that, there's one meaningful change to existing functionality here: it appeared to me that AvroGenerator.writeBinary() was unnecessarily copying the byte[] prior to wrapping it in a ByteBuffer, so I removed this copy.

There's still work to do to add AvroSchemaGenerator support for fixed fields from POJO... This would be a great addition! As we discussed, this will likely involve additional annotation support, so feels beyond my comfort-zone to try to tackle for now.

Finally, apologies for the spurious 'import' reformatting; intellij is quite opinionated, and I didn't feel motivated try to reconfigure tonight. :-P

Hope this helps! 
Quick note here: I'd be ok in adding a new method to specifically handle the fixed case, if that'd make sense instead of checking type here?