Fix crash in all calls to OAuth2::Client#get_token

Anders Carlingrequested to merge
github/fork/anderscarling/patch-1 into 1-4-stable

def build_access_token as not found as it was implemented on OAuth2 instead of OAuth2::Client

This error is quite bad as it will happen on each invocation of OAuth2::Client#get_token and is suppressed by the fact that the call to build_access_token is wrapped in a rescue discarding the exception.

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