Handle requests w/leading slashes throughout api for hosts with faraday configured url prefixes

[alp06h]

Issue is articulated here: lostisland/faraday#293

Issue:
conn = Faraday.new(:url => 'http://example.com/api')
conn.get '/index' #=> GET http://example.com/index

Solution:
conn.get 'index' (remove leading slash)

Example in discourse_api get('categories.json', params) instead of get('/categories.json')

In most cases clients will not experience this as I assume requests from domain ('http://example.com/') would be desired. However, if that is not the case faraday seems to strip url prefix when request has a leading slash.

I altered client.rb in a way that this won't impact any existing discourse_api users, but will not allow the ability to bypass this faraday quirk if they pass fourth argument upon initialization.

[oblakeerickson]

Thank you for spending time on this PR, but I don't think this is something I really want to merge in.

99.9% of discourse installs all use a subdomain instead of inside of a folder off of the domain:

https://meta.discourse.org/t/can-discourse-run-in-a-folder-like-example-com-forum/18303