Ignore read-only attributes when updating through REST resources

[AnthonyRobertson17]

Description

Follow up on the changes in this PR #1149. When a resource has read-only attributes and you attempt to call save, the update payload would contain an erroneous entry for the read-only attributes with the value of HashDiff::NO_VALUE.

The root cause of this issue is that the original_state contains the read-only attributes, however the lookup of the current resource state with to_hash(true) strips them out. Performing a left_diff hash comparison then yields the resulting HashDiff::NO_VALUE.

The fix I've implemented is to use a copy of the original_state hash with read-only attributes removed when performing the comparison to see which attributes to update.

How has this been tested?

I've written a new unit test for the base resource and verified it failed as expected prior to implementing the fix.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[rtymchyk]

Makes sense to me :)