Using '=' to choose arguments breaks multi-line inputs for type 'string'

My code needs to input multi-line strings as a single value.
Entirety of the test code I created to test this issue:

require 'optimist'
opts = Optimist::options do
  opt :foo, 'foobar', short:none, type: :string
end
puts "foo is #{opts[:foo]}"

Command that works:

ruby rubyfile.rb --foo "bar
baz
qux" 

Output of command that works:

foo is bar
baz
qux

Command that produces error:

ruby rubyfile.rb --foo="bar
baz
qux" 

Output of command that produces error:

foo is bar

Start by running the two Ruby command examples from the issue against the provided Optimist option definition and compare how the value is parsed. Trace the handling of the --foo and --foo= forms; done means the equals form preserves all three lines of the multiline string, matching the space-separated form.