Fix handling of rewind after reading all input.

[ioquatix]

Fixes an issue where #rewind no longer works if the entire body is read, causing an implicit close.

Fixes #24.

Types of Changes

• Bug fix.

Contribution

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

[ioquatix]

Without this PR:

> curl -X POST -d "This is test data" http://localhost:9292
First read: "This is test data"

Second read: ""⏎                                                                                                                                                 

With this PR:

> curl -X POST -d "This is test data" http://localhost:9292
First read: "This is test data"

Second read: "This is test data"⏎