Retry http request when unexpected error occurs

[serihiro]

What will this PR change?

• implement Retry fetch_access_token! if connection fails #89
  • retry when request failed or timeout in GCECredentials#fetch_access_token and UserRefreshCredentials#revoke!
  • when give up retrying, raise Signet::AuthorizationError with the string of original_error.inpsect

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[serihiro]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

[serihiro]

@blowmage
Could you review this? I implemented for this issue #89 which you wrote.

[blowmage]

Looks great!

[serihiro]

@blowmage
Thank you fore your feedback. I fixed retry_with_error method and squashed into 960cac2 .
Travis CI job was failed for rubocop error, but I confirmed all specs were passed in my env(ruby 2.3.3p222).

[hxiong388]

@serihiro Thanks for submitting this PR.

Our team has recently taken over the ownership of this repository, and I'm in process of triaging the PRs and issues. I'll merge this PR once you can fix the rubocop violations.

[coveralls]

Coverage increased (+0.03%) to 98.443% when pulling 227ba09 on serihiro:retry-request-with-unexpected-error into 0f7fe1e on google:master.

[serihiro]

@hxiong388
I fixed rubocop violations and rebased master branch.
Could you check this again?

[hxiong388]

@serihiro Thanks! Looks great.