Introduce Base.query_format for URL encoding values

[seanpdoyle]

Follow-up to #421

Problem

Not all HTTP APIs support snake_case query parameters. Active Resource's built-in finders (like .find(:all, params: { … }), .all(params: { … }), where(…)) do not support transforming keys prior to their encoding.

If an API expects camelCase keys (like ?firstName=Matz rather than ?first_name=Matz), Active Resource's does not provide a method or hook to override. The already defined Base.query_string method used by the current implementation is private. If a consumer were to override it (despite depending on or overriding private methods being discouraged), they would need to be responsible for transforming the Hash and encoding it to a String.

Proposal

This commit proposed the introduction of the
ActiveResource::Formats::UrlEncodedFormat. It's modeled after the XmlFormat and JsonFormat, and defines an encode method with the prior behavior (a call to Hash#to_query). Along with the new class, this commit also introduce a new .query_format class attribute (with getter and setter methods), modeled after the .format class attribute.

Consumers can provide their own implementation with or without depending on the ActiveResource::Formats::UrlEncodedFormat. For example, callers can camelCase keys:

module CamelcaseUrlEncodedFormat
  extend self, ActiveResource::Formats::UrlEncodedFormat

  def encode(params, options = nil)
    params = params.deep_transform { |key| key.to_s.camelcase(:lower) }

    super
  end
end

class Person < ActiveResource::Base
  self.site = "https://example.com"
  self.query_format = CamelcaseUrlEncodedFormat
end

Person.where(first_name: "Sean")
 # => GET https://example.com/people.json?firstName=Sean

The URL encoding only applies to query parameters. Prefix options remain snake_case:

class Person < ActiveResource::Base
  self.site = "https://example.com"
  self.prefix = "/teams/:team_id"
  self.query_format = CamelcaseUrlEncodedFormat
end

Person.where(team_id: 1, first_name: "Sean")
 # => GET https://example.com/teams/1/people.json?firstName=Sean

This commit also includes changes to the README.md example code's query parameters to demonstrate that keys are snake_case by default.