Add support for defaults defined in user-specified Avro schema

[JoshRosen]

This PR is based on @granturing's #173, with merge-conflicts fixed by @JoshRosen; the original description is as follows:

The current Avro schema option does not seem to support defaults defined in the user-supplied schema. This is causing some confusion for users who are expecting default values but instead are getting null.

This change forces the user-supplied Avro schema to be used for parsing and converting Avro records to Spark SQL Rows or it'll fall-back to the inferred schema if none is supplied.

This is related to #176

[codecov-io]

Current coverage is 91.46% (diff: 100%)

Merging #195 into master will increase coverage by 0.21%

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[JoshRosen]

This LGTM, so I'm going to merge it in and will cut a release.