Fix FrozenError in Typhoeus streaming response body

[patrickjaberg]

When stubbing a response for the Typhoeus adapter, and the Typhoeus request has an on_body callback, a FrozenError exception is raised when attempting to concatenate the current chunk of the response to the existing response body (i.e. response.body << chunk).

FWIW, my use case for this is to abort a request as early as possible when the response body exceeds a given size, specifically when the response doesn't have a Content-Length header.

The example below illustrates the issue:

require "bundler/inline"

gemfile do
  source "https://rubygems.org"

  gem "typhoeus", "1.4.1"
  gem "webmock", "3.24.0"
end

WebMock.enable!
WebMock.stub_request(:get, "https://example.com").to_return(status: "200", body: "body")

request = Typhoeus::Request.new("https://example.com")

request.on_body do |chunk, response|
  response.body << chunk
end

request.run

This change initializes the Typhoeus response body to a non-frozen, mutable string when using the on_body callback.