Add given httpTransport in GooglePublicKeysManager

[xbauquet]

Construct the default GooglePublicKeysManager with the custom
httpTransport. This avoid error due to proxy.
Fixes #150

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[xbauquet]

I signed it!

[googlebot]

CLAs look good, thanks!

[hiranya911]

LGTM 👍

Thanks @xbauquet for the contribution.

[quackes]

Hi,
when will this be released?

[hiranya911]

I'll try to find some time next week to stage a release.