Improve Do methods

[vthiery]

[This PR is more a proposal than anything else]

Hi!

This PR aims at simplifying the Do methods for both httpClient and hystrixHTTPClient. It also introduce multi-errors for the hystrixHTTPClient.

The reasons why this PR is a proposal more than anything are:

• it changes the way errors are returned by the the hystrixHTTPClient,
• it may slightly break retro-compatibility in case the client would want to use a non-nil response together with a non-nil error. As explained in detail here this might be the case in Go 1.

Let me know what you think 😉

[rShetty]

@vthiery Thanks for this PR. #33 will be merged into master. This PR might have to change

[vthiery]

@rShetty sure! Let me know when #33 gets merged and I'll adapt this PR

[rShetty]

@vthiery This will change after the new major changes to the master, this #33 , Please fix merge conflicts accordingly

[vthiery]

@rShetty I updated the PR. The CI pipeline fails but I believe this is due to a very short timeout used in tests.

[rShetty]

@vthiery Do you want to give a try to fix the conflicts ?

[vthiery]

@rShetty I'm not sure I can marry the valkyrie.MultiError with the recent changes on the hystrix client. Also, for the sake of clarity, I'd prefer to split this PR into two parts:

• idiomatic, cosmetic changes
• introduce valkyrie.MultiError in hystrix client

I can quickly take care of the first point but I may not have the time to work on the second point right now.