* [docs] Clarify SetURLVars
Clarify in documentation that SetURLVars does not modify the given
*htttp.Request, provide an example of usage.
* Short and sweet function doc, example test.
* Add a function to set url params for test
* [docs] add justification for SetURLVars and description of alternative approach to setting url vars.
* rename SetURLParams to SetURLVars as this is more descriptive.
* rename testing to testing_helpers as this is more descriptive.
* [docs] add stipulation to SetURLVars that it should only be used for testing purposes