Moving the `GIT_HTTP_USER_AGENT` handling to ruby method
Moving the `GIT_HTTP_USER_AGENT` handling to ruby method

- By doing it in a ruby method, we can more easily remove it
  then add it back so that it's present for other providers

testfairy/dpl,testfairy/dpl,testfairy/dpl,travis-ci/dpl,travis-ci/dpl,computology/dpl,pengsrc/dpl,travis-ci/dpl