Fix failing test for avro producer

[claudio-benfatto]

This PR fixes the failing test for the Avro producer:

--- FAIL: TestAvroProducer_Add (0.00s)
    avroProducer_test.go:17: Error adding msg: unexpected end of JSON input
    sync_producer.go:115: Expected to exhaust all expectations, but 1 are left.

due to not matching request url with the POST switch case of the createSchemaRegistryTestObject switch

[codecov]

Codecov Report

Merging #8 into master will decrease coverage by 3.92%.
The diff coverage is 22.72%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   61.66%   57.73%   -3.93%     
==========================================
  Files           5        5              
  Lines         253      265      +12     
==========================================
- Hits          156      153       -3     
- Misses         75       90      +15     
  Partials       22       22              

Impacted Files	Coverage Δ
avroConsumer.go	17.24% <ø> (ø)
cachedSchemaRegistry.go	85.10% <ø> (ø)
schemaRegistry.go	78.00% <ø> (ø)
avroProducer.go	36.53% <22.72%> (-18.47%)	⬇️

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