Fixed Collection access in Oauth Plugin

A Collection was being accessed with a built-in array function.
Removed duplicate code for getting params to sign. Made optional params only be signed if they have been set in the config, and not null by default. Added test to check that optional params have not been added if they were not set in the config for the OauthPlugin.
Fixed issue where request params were being copied to the Authorization header. Reverted test to remove incorrect strings, but left order be flexible.
Spelling typo.
CS cleanup