Default value for exc error

[DariuszMusielak]

Description

I would like to introduce default value for exc in def initialize for Faraday::Error.

In some cases it is difficult to find the reason on a first spot when doing raise Faraday::ConnectionFailed because it raises ArgumentError. In other cases like raise Faraday::TimeoutError it works because Faraday::TimeoutError sets default value or exc.

Link to related issues if any. (As Fixes #XXX) - there was no related issue.
I found out that someone has been asking why this is happening on Stackoverflow, I spent some time today looking for an answer for myself.

From my perspective introducing default value does not break anything and simplify codebase.

Todos

List any remaining work that needs to be done, i.e:

• Tests
• Documentation

Additional Notes

Optional section
Stackoverflow: https://stackoverflow.com/questions/59176256/unexpected-behaviour-with-faradayconnectionfailed/70427415#70427415

[iMacTia]

I couldn't find a reason for why the exc parameter was enforced. We may argue it's not a good idea to raise an exception without useful information in it, but going as far as making it manadatory makes testing more complicated than necessary (as this PR shows fixing our own tests).

LGTM 💯