Avoid data race when using http.DefaultClient (#78)

[grrru]

When no custom HTTPClient is provided, BuildHTTPClient() returns http.DefaultClient.
Later, addRedirectFunctionality sets client.CheckRedirect on this shared instance.

Under concurrent usage, multiple goroutines mutate DefaultClient.CheckRedirect at the same time, causing a data race.

[grrru]

@levigross
Hello. Can I ask you to review this?

[levigross]

This will cause a lot of copies of the http.Client per request. I would suggest changing the addRedirectFunctionality code 😃

[grrru]

@levigross
Hi.
I updated the PR and modified addRedirectFunctionality as suggested to avoid in-place mutation.

However, in my opinion, The revised patch does not seem to meaningfully reduce http.Client copies, and allocation behavior appears largely unchanged.

Could you please confirm if this is the direction you intended, or advise on a better approach?