Use IDbConnection instead of DbConnection.

[moonheart]

I think we should depends on interfaces rather than concrete classes. How do you think😄

[codeapologist]

My initial implementation actually used IDbConnection, but I later discovered that Dapper's internal implementation is coupled with DbConnection object. So I made a change to the accept DbConnection to ensure working functionality. You can see the release note here: https://www.nuget.org/packages/DataAbstractions.Dapper/1.0.2

[moonheart]

I see. The IDbConnection does not have an OpenAsync method which 'SqlMapper.Async' depends on.
As these issues descripted https://github.com/dotnet/corefx/issues/3480#issuecomment-159533059, DapperLib/Dapper#757
Thanks for your detailed explanation, I'll close this PR now.

[codeapologist]

I had a change of heart on this. Dapper is handling OpenAsync appropriately when provided a DbConnection vs IDbConnection, and it is the developer's responsibility to use the connection object correctly. So with that being said, I have updated the constructor signature to accept IDbConnection instead of DbConnection in version 3.1.0. This isn't a breaking change.