ApiVersion::NullVersion tweaks

[jtgrenz]

1. Not strictly a bug, but Session defaults api_version to nil whereas Base defaults it to ApiVersion::NullVersion. For consistency sake, lets keep them the same.

2. Weird edge case, but if you for some reason stored the value of Base.api_version and then find that version stored, you'd create an ApiVersion called NullVersion. Again, maybe not strictly a bug, but I don't think anyone would intentionally try to find NullVersion and intend to create it. The following example seems like a legit stumbling block, so lets smooth the road so to speak.

ShopifyAPI::ApiVersion.version_lookup_mode
# => :define_on_unknown
ShopifyAPI::Base.api_version
# => ShopifyAPI::ApiVersion::NullVersion
original_version = ShopifyAPI::Base.api_version
ShopifyAPI::Base.api_version = '2019-07'
# => "2019-07"
ShopifyAPI::Base.api_version = original_version
# => ShopifyAPI::ApiVersion::NullVersion
ShopifyAPI::Base.api_version
# => #<ShopifyAPI::ApiVersion:0x00007f835bb2c150 @display_name="ShopifyAPI::ApiVersion::NullVersion", @handle="ShopifyAPI::ApiVersion::NullVersion", @latest_supported=false, @supported=false, @verified=false>

This PR prevents NullVersion from being instantiated and changes find_version to raise if passed the NullVersion class.

[alexaitken]

api_version= should work with nil, ApiVersion::NullVersion, any ApiVersion and any String that is in an api version format.

Right now passing in ApiVersion::NullVersion will raise. Making the below code impossible.

session_1= Session.new(domain: domain, url: url, api_version: nil)
session_2 = Session.new(domain: session_1.domain, url: session_1.url, api_version: session_1.api_version)

[jtgrenz]

guess this is a classic case "if its not broken, don't fix it". Session having a nil version has no negative affects as far as I can tell. Base having a nil version caused us issues, hence the NullVersion. I'll just remove this first commit.

[alexaitken]

I think the coercion to out of primitives to ApiVersions is the right move.

This type of change should cover us.

@api_version = ApiVersion::NullVersion.matches?(version) ?  ApiVersion::NullVersion : ApiVersion.find_version(version)




class NullVersion
  def matches?(version)
    version.nil? || version == self
  end
end

[jtgrenz]

That sounds like a clean solution to me. Thanks! I just pushed it.