Fix duplicate method warning when requiring httpx

Earlopainrequested to merge
Earlopain/httpx:redefinition-resolver-class into master

From d67d705a

irb(main):001> require "httpx"
/home/user/code/httpx/lib/httpx/options.rb:461: warning: method redefined; discarding old option_resolver_class
/home/user/code/httpx/lib/httpx/options.rb:407: warning: previous definition of option_resolver_class was here

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