Prevent modification of attribute defaults

[mblythe86]

Description

Either freeze the default attribute values (in the case of String or empty Hash/Array) or call a Proc to return a unique instance of a mutable object (mostly relevant for the principals attribute of PermissionTarget). Add a check in the attribute method to enforce this.

Receiving a FrozenError in this case may be unexpected for users, but it will force them into the preferred method of setting attributes (i.e. using the #{name}= setter).

Related Issue

Fixes issue #174.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change) (This possibly breaks any code that doesn't use the #{name}= setters and relies on this undocumented behavior.)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly. (I don't think any documentation needs to be updated?)
• I have added tests to cover my changes.
• If Gemfile.lock has changed, I have used --conservative to do it and included the full output in the Description above. (No changes to Gemfile.lock)
• All new and existing tests passed.
• All commits have been signed-off for the Developer Certificate of Origin.

[mblythe86]

Why freeze these instead of creating a Proc that would return an empty Hash?

Because the Proc approach could silently drop data. For example, if a user did this:

build = Artifactory::Resource::Build.new
build.properties['key'] = 'value'
puts build.properties

It would print {} instead of {"key" => "value} like a user would expect. This happens because the user modified the mutable default returned by the getter, not an object stored in the object's attribute hash.

If this method of accessing and modifying attributes is common, then the getter could be enhanced to store that default value into the object's attribute hash before returning it.