Represent empty body response as blank string.

[ebababi]

Faraday adapters are expected to represent empty body response as blank string and this PR aligns async-http-faraday to this specification, fixing #46.

Moreover, the returned blank string is unfreezed, in the same way the "reference implementation" of the faraday-net_http adapter is doing it.

Last, here's a third-party implementation that I've stumbled upon that uses Faraday through the ruby-openai gem and always expects a string response.

Types of Changes

• Bug fix.
• Breaking change.

Not sure how you would categorize this change.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

@iMacTia what about allowing the response body to be frozen, it's more efficient - immutable should be the default if possible.

[iMacTia]

The reason we need it unfrozen is because we need to control the encoding of the string, BUT you should be able to return a frozen string with no issues: we check if the body is frozen shortly afterwards and call #dup on it if that's the case (see here).

Returning an already unfrozen string might be more efficient though to avoid Faraday dup'ing a really big response body

[ioquatix]

body.dup should be efficient for frozen strings as it should internally use copy-on-write.

[iMacTia]

In that case then I'd expect you to be able to set the body to a frozen string and Faraday should just #dup it when needed