Do not ignore avro files without extensions by default

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to change the default behaviour of AVRO datasource which currently ignores files without .avro extension in read by default. This PR sets the default value for avro.mapred.ignore.inputs.without.extension to false in the case if the parameter is not set by an user.

How was this patch tested?

Added a test file without extension in AVRO format, and new test for reading the file with and wihout specified schema.

[codecov-io]

Codecov Report

Merging #286 into master will increase coverage by 0.33%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
+ Coverage   92.23%   92.56%   +0.33%     
==========================================
  Files           5        5              
  Lines         322      323       +1     
  Branches       43       41       -2     
==========================================
+ Hits          297      299       +2     
+ Misses         25       24       -1

[gengliangwang]

Left some comments in https://github.com/apache/spark/pull/21769 .

[MaxGekk]

@gengliangwang I ported the commit https://github.com/apache/spark/commit/ba437fc5c73b95ee4c59327abf3161c58f64cb12 . Please, take a look at the PR again.

[gengliangwang]

Thanks, merge to master.