[Ruby] Fix "build_from_hash" function for Ruby enums

[run-o]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Modify build_from_hash to compare the given value against the enum's value rather than the enum's constant name.
Unless I'm missing something, it seems to me that this should be the desired behavior for build_from_hash.

For instance, consider the following Ruby enum class:

class Enum
  ENUM1 = "enum 1"
  ENUM2 = "enum 2"
  ENUM3 = "enum 3"
  def build_from_hash(value)
    ...
  end
end

If we run the following:

Enum.new.build_from_hash("enum 2")

It'll return "enum 2".
Before this fix, it would return an error since we were comparing "enum 2" against the constant names "ENUM1", "ENUM2" and "ENUM3".

Hope this makes sense!

[run-o]

@wing328 Here's my PR for the Ruby enum fix. Let me know if it makes sense. Thanks!

[wing328]

@run-o thanks for the PR. Can you add a test case in a new file named "enum_spec.rb" under the spec folder for the following enum class:

 module Petstore
   class EnumClass
     
     ABC = "_abc".freeze
     EFG = "-efg".freeze
     XYZ = "(xyz)".freeze

[wing328]

cc @NicoEigenmannCW

Linked to #6131

[wing328]

@run-o please add the test case later when you've time. Thanks.

The fix looks good to me so merging it into master.